I'm experiencing the same problems on 2 machines with 12.04 (with beta2 AND yesterday final release); MP4 files don't play in default movie player and mp3 quality and timing seem poor. Installing medibuntu packages and restricted extras didn't solve.
The only player that seems to work fine for me is VLC.
I didn't have these problems in 11.04 and 11.10
file
command to find information about the format and encoding of files of many types, including most audio/multimedia types. For more info one getting information about media and/or other files, I recommend posting a new question. – Eliah Kagan Jun 02 '12 at 20:20