As far as I know Musepack isn't a proprietary format like MP3 and its GPL-ed. I have found on google a while ago that the new GStreamer has built in support for .mpc files. Would it be possible to make Rhythmbox to play those files using the new GStreamer framework - I like Rhythmbox much more than XMMS, but I can't use it due to the lack of .mpc support :-(