Have been looking for a similar solution for some time now. Closest that I have managed to get so far is software for a PDA running on a Wi-Fi network which can remotely control my PC.
Your media library is downloaded to the PDA so you can browse select and play. Been playing with it for a week or so now and although there are some bugs it does provide a better solution than firing up all your AV gear just to play some music.
Please not that I have not tested it with a Xbox360 primarly as my xbox will stop playing streamed music after about 20mins and Microsoft cannot provide a fix??!!?
Please look at the following website[Link: rudeo.com]