Some info: The only way to change the audio volume, is by changing it's waveform. As the mp3 files do not contain a pure waveform, it is impossible to change an mp3's volume without first converting it to a wave file and afterwards recompressing it to mp3. Thus, when saving an mp3 with changed volume you have to be carefull with the output bitrate. Even the progs that do not give you a bitrate option do have an internal encoder with probably a fixed bitrate.
That beeing said, I'd suggest you downloaded the shareware version of acoustica 3.3 from tucows.com to do your job.
I hope this was of help.