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: 
The most reliable way to accomplish this is to use the Windows Mixer to change your recording source. 

1. In Windows XP, open the mixer (Windows+R "sndvol32").
2. Go to Options > Properties.
3. Change the radio button from Playback to Recording.
OR, if Recording is grayed out, your sound driver enumerates the recording properties on a different device - change the Mixer Device at the top. On my laptop, it's "Realtek HD Audio input."
4. You should see some options like Line, Mic, Stereo Mix (or "What-U-Hear" if you have a Creative Labs device). Make sure they are all checked.
5. Say OK, and in the mixer, you can now select your recording source with the checkboxes along the bottom.
6. Change it to the Stereo Mix/What-U-Hear/etc. option.
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!