Absolutely! Integrating your computer into your inwall or in-ceiling sound system is simpler than you think.
You will need a small amplifier or basic stereo receiver to boost the signal that comes from your computer's sound-card. To link that to your computer, you will need an adaptor.
Generally, a 1/8" 2-Male RCA adaptor would work for this application; it will go from your computer's sound card —from the 'line out' to one of the inputs on the receiver/amp.
You can find them on Amazon.com
Belkin Audio Y Cable Splitter 1-Mini Plug, 2-RCA Plugs (6 feet)
Another way to play MP3's is via Apple's AirPort Express system explained here on Inwallblog.com:
The speakers would then be connected to the 'speaker out' jacks from the receiver or amplifier. You can also hook up a powered subwoofer from there as well.
OR… you can integrate a computer into your stereo/entertainment center by using the same adaptor. You will probably have to have your computer near the entertainment center in order to do this, but it's a viable option for those who want to include their home computer in their home entertainment system.
Doing this can take the sound from your computer gaming to a whole new level, and give you the ability to listen to music in an array of formats, such as MP3 and more! It's a nice way to use a computer's capabilities for enhancing sound.