Question: In a chat session, whenever I want the other person to hear a pre-recorded voice, I need to play it on my speakers and use my microphone to capture it. How can I play MP3 files into the microphone input so that whenever I do a voice chat I could play the MP3 file in such a way that I am speaking through the microphone itself?

Operating system: XP Pro sp3

Answer: 
@Rook has a point...but if that doesn't work or suit you, here is what I suggest.

I would take a regular 3.5 mm stereo cable ([see here on Amazon.com](http://rads.stackoverflow.com/amzn/click/B00021ENXM)), plug one end into the headphone jack of your computer (the output) and then plug the other end into your microphone (input) port. Then set the source to be the microphone jack, and just play the MP3 with your favorite music program (Zune software, Windows Media Player, WinAmp, etc) and that should do the trick....

You could also just get your MP3 player, plug one end (output) into the headphone jack of the MP3 player, and the other end into the microphone (input) jack...that will do the same thing...
I do recommend using the Jack too as stated before but it would be a problem to keep changing the sound settings of your recording and playback device and chat at the same time, but if you need any software kindly search for this one "Many Cam Virtual Webcam 3.1.43" which play a role as a virtual Mic and Virtual Cam, I hope that would help!