Replay Gain <http://replaygain.hydrogenaudio.org/>

You might be able to use foobar2000 to just knock these guys out: scan them, and then use the diskwriter (applying replaygain) to put them back out as waves, and then recode into your preferred format. If the source files are already encoded, you might be able to tweak the source code of mp3gain, vorbisgain, wavgain to set to the SPL you're looking for.

--Nathan