Maybe you're already well aware of this, but there's a guy out there who has been trying to develop a standardized method of dealing with this problem of apparent volume differences between individual MP3 songs.

His project is at this URL:

http://privatewww.essex.ac.uk/~djmrob/replaygain/contents.html

He already wrote some Windows software that performs statistical analysis on an MP3 file compared to pink noise and comes up with the correct value to achieve a standard 83db of gain for the file. It looks like he's been trying since mid 2001 to get people to adopt his standards into an MP3 player program.