SOUND: Add MP3 detection

We determine that a stream is a MP3 stream if
a) We find an ID3v2 tag   or
b) We find a sane MPEG frame header

This is not fool proof, but should suffice for us.
Should false positives become a problem later one, we should
a) Calculate frame lengths and check next $n frames   and
b) Check the ID3v2 tag for sanity   and
c) Also check for sane frame headers when we found an ID3v2 tag