When I adjust the amplitude of a song in Soundtrack Pro up or down by, say 6 dB, and then convert the sound file from wave to mp3, I notice that the volume increase or decrease is noticeable in the wave version but not in the mp3 version. Although the mp3 is a compressed version of the song, I didn't expect the volume changes to be completely bypassed or filtered out.

Any tips on how to increase or decrease song volumes and to maintain these volume changes within the mp3 format?

Thanks in advance.