just use this link to download the MP3:
http://translate.google.com/translate_tts?tl=el&q=%22hello%22
Edit:
Be aware Some Browsers like Firefox replaces the %22 with " and the link doesnt work so you need to copy it and cant click it!
How to Download Files:
http://www.csharp-examples.net/download-files/
http://msdn.microsoft.com/en-us/library/ez801hhe.aspx
How to Play MP3:
http://msdn.microsoft.com/en-us/library/4y171b18.aspx
using System.Media;
String strTextYouWantAsMp3 = "Hello";
WebClient webClient = new WebClient();
webClient.DownloadFile("http://translate.google.com/translate_tts?tl=el&q=%22" + strTextYouWantAsMp3 + "%22", @"c:\audio.mp3");
SoundPlayer simpleSound = new SoundPlayer(@"c:\audio.mp3");
simpleSound.Play();