Are you expecting to have to use your computer to control the music anyway? You can just get a BT receiver for like $20 (iirc) on Amazon and it does just what you want...but you have to control it all from your computer but it also leaves your phone free.
I can't think of anything that
pushes music to the phone though...I do not think that is possible.
Your other option would be to find an app that uses your desktop as a server for your music. So this would be your phone
pulling music. This is what PLEX does.
Plex | Windows Phone Apps+Games Store (United States)