Try the application MP3Gain at this site:
http://www.geocities.com/mp3gain/
It allows you to increase the volume on mp3 files. WARNING: Before running this, copy the mp3's you want to boost to another directory. This program will alter the selected mp3 files. It will NOT make copies of them. If you mess around with the originals, you might damage them, and then not be able to recover them.
Hope this helps!