You may be able to get a microphone that connects to a USB port (like the ones you can use to connect to the Playstation 2 for Karaoke Revoution) that may go through your speakers (I could be wrong though, I am just guessing).
Also, I am sure you can buy a jack that can plug into your computer, and then you can plug your microphone into that jack.
Again, not too sure, just trying to help