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 have a solution that would serve your needs:

"MorhpVOX Pro" has an option to transmit your desired audio files through microphone and others can hear you aswell, so it would seem as if the music is playing (quality of the transmitted audio doesn't depend on your microphone) in the background.

The stereo mix option doesn't do exactly that, rather it will play all sounds you hear, so it doesn't serve the purpose you're looking for, bec it will play all the damn sounds, even others voices when they speak, so yeah... not good.