There's 2 main types of mp3 files that I know of. One is the regular mp3 format & the other is mp3Pro which is really only effective for very low bit rates. I'd suggest you find one of those with the raphsody icon & change the default for it. Setting default audio program

However, there may be specific program type files that show up as mp3 but are really a more specific format such as Realplayer files or similar. Another possibility is it's one of those very rare formats that allows bit rates larger than 320. AFAIK I have never come across mp2 format on Gnutella but they may well show up as mp3 I don't know. mp2 format was popular before mp3 was started.