CapMonster 2 software starting from 2.5.0.0 version allows to recognize old and new audio ReCaptcha. We have developed special module ZennoLab.AudioReCaptcha for this. When you get ReCaptcha on a webpage, you should select audio recognition and send it to CapMonster.

Here is the scheme:

To send audio ReCaptcha from ZennoPoster (v.5.9.5.1 and higher) you should use the following snippet. Here is the code:

//Main settings
 
// waiting timeout
var waitTime = 1000;
// recognition attempts
var tryRecognize = 10;
// attempts to load element
var tryLoadElement = 60;
// show recognizing progress messages
var needShowMessages = false;
// need to check answer's correctness
var needToCheck = true;
 
// Additional variables
 
// tab
var tab = instance.ActiveTab;
// congratulations, you are not robot
var success = false;
// audio file
var file = string.Empty;
// file has been deleted
var fileDeleted = true;
// answer to audio captcha
var answer = string.Empty;
// ReCaptcha 1
var isRecaptcha1 = false;
// timeout exceeded 
var timeout = false;
// error loading file
var fileLoadingError = false;
var usedURLs = new List<string>();
var isFirstAttempt = true;
var coincidenceReCaptcha2Index = -1;
 
// Searching audio button element
Action TryOpenAudioForm = () => {
	for (int k = 0; k < tryLoadElement; k++)
	{
		var audioButton = tab.FindElementById("recaptcha-audio-button");
		// if found
		if (!audioButton.IsVoid)
		{
			// press button
			audioButton.Click();
			tab.WaitDownloading();
			break;
		}
		System.Threading.Thread.Sleep(waitTime); // pause
		if (k == (tryLoadElement - 1)) timeout = true;
	}
	System.Threading.Thread.Sleep(waitTime); // pause
};
 
Action OpenAudioTask= () => {
	project.SendInfoToLog("Loading audio task", needShowMessages);
	//checking task type in the form
	var task = tab.FindElementByAttribute("div", "class", "rc-imageselect-desc-wrapper", "regexp", 0);
	if (!task.IsVoid) TryOpenAudioForm(); // graphic task has been found, then loading audio
};
 
Action VisibleIndexReCaptchaDefinition= () => {
	var recaptchaElementsGroup = tab.FindElementsByAttribute("div", "class", "recaptcha-checkbox-checkmark", "regexp");
	int length = recaptchaElementsGroup.Elements.Length;
	if (length == 1)
	{
		coincidenceReCaptcha2Index = 0;
		return;
	}
 
	for(int i = 0; i < length; i++)
	{
		var element = recaptchaElementsGroup.Elements[i];
		if (!element.IsVoid)
		{
			int x = element.DisplacementInTabWindow.X;
			int y = element.DisplacementInTabWindow.Y;
 
			var suspectVisibleElement = tab.GetElementFromPoint(x, y).DisplacementInTabWindow;
			if (x == suspectVisibleElement.X && y == suspectVisibleElement.Y && element.Width != 0 && element.Height != 0 && x != 0 && y != 0)
			{
				coincidenceReCaptcha2Index = i;
				break;
			}
		}
	}
};
 
// Identifying ReCaptcha version
Action NotRobotClick= () => {
	for (int k = 0; k < tryLoadElement; k++)
	{
		VisibleIndexReCaptchaDefinition();
		if (coincidenceReCaptcha2Index < 0) coincidenceReCaptcha2Index = 0;
 
		// поиск кнопки "Я не робот"
		HtmlElement notRobot = tab.FindElementByAttribute("div", "class", "recaptcha-checkbox-checkmark", "regexp", coincidenceReCaptcha2Index);
 
		// кнопка существует
		if (!notRobot.IsVoid)
		{
			// клик по кнопке
			notRobot.Click();
			System.Threading.Thread.Sleep(waitTime); // подождём немного
 
			// если ввод каптчи не требуется
			var check = tab.FindElementByAttribute("div", "class", "recaptcha-checkbox-checkmark", "regexp", coincidenceReCaptcha2Index);
			if (!check.IsVoid)
			{
				if (check.OuterHtml.Contains("style=\"\""))
				{
					success = true;
					timeout = false;
					break;
				}
			}
		}
 
		// Форма существует
		var loadedForm = tab.FindElementByAttribute("div", "class", "primary-controls", "regexp", 0);
		if (!loadedForm.IsVoid)
			break;
 
		// checking if ReCaptcha 1 audio button exists
		HtmlElement r1 = tab.FindElementById("recaptcha_switch_audio");
		if (!r1.IsVoid)
		{
			r1.Click();
			isRecaptcha1 = true;
			break;
		}
 
		// подождём загрузки элементов
		System.Threading.Thread.Sleep(waitTime);
		if (k == (tryLoadElement - 1)) timeout = true;
	}
};
 
// Downloading mp3 file
Action GetAudioFile= () => {
 
	var href = String.Empty;
 
	// getting audio task
	for (int k = 0; k < tryLoadElement; k++)
	{
		if (isRecaptcha1)
		{
			var audioChallenge = tab.FindElementByAttribute("a", "id", "recaptcha_audio_download", "text", 0);
			// if found, getting link to audio file
			if (!audioChallenge.IsVoid)
			{
				href = audioChallenge.GetAttribute("href");
				break;
			}
			else System.Threading.Thread.Sleep(waitTime);
		}
		else
		{
			if (isFirstAttempt)
			{
				isFirstAttempt = false;
			}
			else
			{
				var reload = tab.FindElementById("recaptcha-reload-button");
				if (!reload.IsVoid)
				{
					reload.Click();
					tab.WaitDownloading();
				}
			}
 
			var audioChallenge = tab.FindElementByAttribute("a", "class", "rc-audiochallenge-tdownload-link", "regexp", 0);
			// if found, getting link to audio file
			if (!audioChallenge.IsVoid)
			{
				href = audioChallenge.GetAttribute("href");
				break;
			}
			System.Threading.Thread.Sleep(waitTime); // waiting when form will be loaded
		}
		if (k == (tryLoadElement - 1)) timeout = true;
	}
 
	foreach (var usedUrl in usedURLs)
	{
		if (usedUrl.Contains(href))
			throw new Exception("new audio file is absent");
	}
	usedURLs.Add(href);
 
	project.SendInfoToLog("Downloading audio file", needShowMessages);
	try
	{
		var proxy = instance.GetProxy();
		var respType = ZennoLab.InterfacesLibrary.Enums.Http.ResponceType.File;
		var timeoutRequest = 30000;
		var cookies = instance.GetCookie("google.com", true);
		var userAgent = project.Profile.UserAgent;
		var maxRedirectCount = 1;
		var downloadPath = project.Directory + "\\audiofiles";
		file = ZennoPoster.HttpGet(href, proxy, "UTF8", respType, timeoutRequest, cookies, userAgent, false, maxRedirectCount, null, downloadPath);
		fileDeleted = false;
	}
	catch (Exception ex)
	{
		throw new Exception("Failed to download audio file");
	}
};
 
Action Recognize= () => {
	project.SendInfoToLog("Recognizing", needShowMessages);
	try 
	{
		if (!File.Exists(file)) 
		{
			fileLoadingError = true;
			fileDeleted = true;
			return;
		}
		var bytes = File.ReadAllBytes(file);
		if (bytes.Length < 1024)
		{
			fileLoadingError = true;
			return;
		}
 
		var hexStringBuilder = new StringBuilder(2);
		for (var i = 0; i < 2; i++)
			hexStringBuilder.AppendFormat("{0:x2}", bytes[i]);
 
		var header_prefix = hexStringBuilder.ToString();
		if (!header_prefix.Equals("ffe3"))
		{
			fileLoadingError = true;
                        throw new Exception("Your IP was banned!");
			return;
		}
 
		string str = Convert.ToBase64String(bytes);
		var rc = ZennoPoster.CaptchaRecognition("CapMonster2.dll", str, "CapMonsterModule=ZennoLab.AudioReCaptcha&ParallelMode=true");
		var split = rc.Split(new [] {"-|-"}, StringSplitOptions.RemoveEmptyEntries);
		if (split.Length > 0)
			answer = split[0];
        if (answer.Contains("133193320491090004119440")) throw new Exception("Your IP was banned!");
	} 
	finally 
	{
	    if (File.Exists(file)) 
	    {
			File.Delete(file);
			fileDeleted = true;
        }
	}
};
 
Action InputAnswer= () => {
	if (!String.IsNullOrEmpty(answer) && answer != "sorry")
	{
		project.SendInfoToLog("Inserting answer", needShowMessages);
		HtmlElement audioAnswerInput = null;
		// searching answer input field
		if (isRecaptcha1)
		{
			audioAnswerInput = tab.FindElementByAttribute("input:text", "id", "recaptcha_response_field", "text", 0);
		}
		else
		{
			audioAnswerInput = tab.FindElementByAttribute("input:text", "id", "audio-response", "regexp", 0);
		}
 
		// if found
		if (!audioAnswerInput.IsVoid)
		{
			// inserting answer
			audioAnswerInput.SetValue(answer, "None", false);
		}
 
		// pause
		System.Threading.Thread.Sleep(waitTime);
	}
	else throw new Exception("Answer not received");
};
 
// Verifying
Action SubmitAnswer= () => {
	// searching "Verify" button
	HtmlElement apply = tab.FindElementById("recaptcha-verify-button");
 
	if (!apply.IsVoid) apply.Click();
	tab.WaitDownloading();
 
	// pause
	System.Threading.Thread.Sleep(waitTime);
};
 
// Checking if answer is correct for ReCaptcha 2
Action Check= () => {
	project.SendInfoToLog("Verifying answer", needShowMessages);
	tab.WaitDownloading();
 
	for (int k = 0; k < tryLoadElement; k++)
	{
		System.Threading.Thread.Sleep(waitTime);
 
		var checkAnswer = tab.FindElementByAttribute("div", "class", "recaptcha-checkbox-checkmark", "regexp", coincidenceReCaptcha2Index);
 
		// проверка исчезновения формы
		var loadedForm = tab.FindElementByAttribute("div", "class", "primary-controls", "regexp", 0);
		if (loadedForm.IsVoid)
		{
			success = true;
			break;
		}
		else
		{
			int xPrimaryControlsDisplaysment = loadedForm.DisplacementInTabWindow.X;
			int yPrimaryControlsDisplaysment = loadedForm.DisplacementInTabWindow.Y;
 
			if (xPrimaryControlsDisplaysment < 0 || yPrimaryControlsDisplaysment < 0) // there are no visible recaptcha
			{
				success = true;
				break;	
			}
 
			if (checkAnswer.IsVoid)
				break;
		}
 
		var recaptchas = tab.FindElementsByAttribute("div", "class", "recaptcha-checkbox-checkmark", "regexp");
		if (recaptchas.Count == 0)
		{
			success = true;
			return;
		}
 
		if (!checkAnswer.IsVoid)
		{
			if (checkAnswer.OuterHtml.Contains("style=\"\""))
				success = true;
			else
				success = false;
			return;
		}
		if (k == (tryLoadElement - 1)) timeout = true;
	}
};
 
// Checking "I'm not robot" checkbox
NotRobotClick();
if (success) return "ok";
 
// trying to recognize ReCaptcha
for (int i = 0; i < tryRecognize; i++)
{
	if (!isRecaptcha1) OpenAudioTask();
	if (timeout) break;
	GetAudioFile();
	if (timeout) break;
 
	Recognize();
	if (fileLoadingError) 
	{
		fileLoadingError = false;
		continue;
	}
	if (timeout) break;
	InputAnswer();
	if (timeout) break;
	if (!fileDeleted)
	{
		if (File.Exists(file))
		{
			File.Delete(file);
			fileDeleted = true;
		}
	}
 
	if (isRecaptcha1) return "ok";
	SubmitAnswer();
 
	if (!needToCheck) return "ok";
 
	if (timeout) break;
	Check();
	if (success) return "ok";
}
 
if (timeout) throw new Exception("Loading element timeout is exceeded");
else throw new Exception("Not recognized, Attempts to recognize were ended, before the response was received");

Snippet works with old and mobile user agents in browser.
Changing proxy is obligatory as ReCaptha denies access to audio file after recognizing 3-5 captchas from the same IP-address.
If captcha was not recognize at first attempt, the snipppet will be executed second time. Number of attempts to load elements, waiting timeout, recognition attempts and setting: need to check correctness of CapMonster2 answer, can be set using variables:

// waiting timeout
var waitTime = 1000;
// number of recognition attempts
var tryRecognize = 10;
// number of attempts to load elements
var tryLoadElement = 60;
// need to check answer's correctness
var needToCheck = true;

Here is the function which sends captcha to service:

ZennoPoster.CaptchaRecognition("CapMonster2.dll", str, "CapMonsterModule=ZennoLab.AudioReCaptcha&ParallelMode=true");