Here's a program I'm quite familiar with. Just plain old command line
"normalize" which is in Synaptic. Works with mad or lame for mp3's.
But you have to do it on files on the HD.
I had several directories I was having problems with and ran normalize
from the command line at a -20 setting, the clipping setting is set at default
-6. Voila, no more volume setting problems. But hardware could cause
low volumes on playback, not the files themselves.
Some computers have better speakers than others, but the headphones
at a -20 setting seem to be quite equal, averaged, loud without distortion,
Try it on a few files, if you go up to -15 volume increases, but so does
clipping activity distortion.