Script.cs

using System;
using System.Collections;
using System.Xml.Linq;
using UnityEngine;
using UnityEngine.Networking;

public class TextToSpeech : MonoBehaviour {

    public static readonly string accessUri = "https://api.cognitive.microsoft.com/sts/v1.0/issueToken";
    public static readonly string synthesizeUri = "https://speech.platform.bing.com/synthesize";
    public string textToSpeak = "Oh yeah! Finally Dorot is speaking.";
    private static string accessToken;
    private static readonly string apiKey = "MY-SUPER-DUPER-KEY";

    public TextToSpeech()
    {
    }

    public void Speak()
    {
        Debug.Log("Speaking...");
        StartCoroutine(RequestToken());
    }

    private string GenerateSsml()
    {
        var ssmlDoc = new XDocument(
                          new XElement("speak",
                              new XAttribute("version", "1.0"),
                              new XAttribute(XNamespace.Xml + "lang", "en-US"),
                              new XElement("voice",
                                  new XAttribute(XNamespace.Xml + "lang", "en-US"),
                                  new XAttribute(XNamespace.Xml + "gender", "Male"),
                                  new XAttribute("name", "Microsoft Server Speech Text to Speech Voice (en-US, BenjaminRUS)"),
                                  this.textToSpeak)));
        return ssmlDoc.ToString();
    }

    public IEnumerator RequestToken()
    {
        var tokenRequest = UnityWebRequest.Post(accessUri, "data");
        tokenRequest.SetRequestHeader("Ocp-Apim-Subscription-Key", apiKey);

        var tokenResponse = tokenRequest.SendWebRequest();
        yield return tokenResponse;

        if (tokenRequest.isHttpError)
        {
            Debug.LogError("HTTP Error: " + tokenRequest.error + " Code: " + tokenRequest.responseCode);
        }
        else
        {
            var postStringData = GenerateSsml();
            accessToken = tokenRequest.downloadHandler.text;
            Debug.LogError("Access token: " + accessToken);
            Debug.LogError("postStringData: " + postStringData);

            StartCoroutine(Synthesize(postStringData, accessToken));
        }
    }

    public IEnumerator Synthesize(string text, string token)
    {
        var synReq = UnityWebRequest.Post(synthesizeUri, text);
        synReq.SetRequestHeader("Content-Type", "application/ssml+xml");
        synReq.SetRequestHeader("X-Microsoft-OutputFormat", "riff-16khz-16bit-mono-pcm");
        synReq.SetRequestHeader("X-Search-AppId", "07D3234E49CE426DAA29772419F436CA");
        synReq.SetRequestHeader("X-Search-ClientID", "1ECFAE91408841A480F00935DC390960");
        synReq.SetRequestHeader("User-Agent", "Dorot");
        synReq.SetRequestHeader("Authorization", "Bearer " + token);

        var synRes = synReq.SendWebRequest();
        yield return synRes;

        if (synReq.isHttpError)
        {
            Debug.LogError("HTTP Error: " + synReq.error + " Code: " + synReq.responseCode + " isNetworkError: " + synReq.isNetworkError + " isDone: " + synReq.isDone);
        }
        else
        {
            Debug.LogError("done!");
        }
    }
}