If you have encoded live audio, or have encoded from older cd's you may find variations in the overall sound level.
To change the encoded volume levels of the MP3's you will need to normalise them using wavnorm.
http://www.zog.net.au/computers/wavnorm/
Sox is a very handy sound conversion utility which I'd recommend having, and you will need it if you wish to use wavnorm or re-encode your files.
ftp://sunsite.unc.edu/pub/Linux/apps/sound/convert/
You may also need a mixer program; Xmixer works well and is included with most distributions.