Media playback on emulator should match Phone
The phone currently plays a large selection of media formats (MP4, M4V, 3GP etc) that aren't supported in the emulator. This makes apps that use these formats extremely hard to debug.
Would be awesome if you could fix this :)
Peter Torr commented
FYI, MP4 and AAC will be supported in the emulator - Vibhor covers this in his tools talk (http://channel9.msdn.com/events/mix/mix11/DVC17)