I have posted some code to my site demonstrating the use of NSMovieView to play an MP3 file. It also demonstrates a workaround to an annoying bug, which causes NSMovieView to simply stop playback at arbitrary points in the song. You can download the code and read some addtional background here:

http://www.scifihifi.com/weblog/mac/NSMovieViewAndMP3s.html

– BuzzAndersen