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: 
If your sound card supports it, you can use the Stereo Mix audio input device to stream what comes out of your speakers to someone. You'd have to go into your application's settings (like Skype's settings) and change the audio input from "Integrated Microphone" (or something similar) to "Stereo Mix".
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!