Man, this had me stumped. It started with two mp3 files with greatly differing volumes. The quieter one had been manipulated with MP3Gain. I reapplied MP3Gain at 95db. No effect. After playing around a bit, I noticed that the db changes weren't affecting the volume at all!

At this point I was cursing MP3Gain...then I tried another mp3 player and the differences were apparent. I didn't know Foobar had an automatic volume normalizer! Don't get me wrong, that's a really nice feature, but it left me stumped.

Now the only thing that stumps me is, why were these two files playing at different volumes to begin with?

I love foobar. Any chance of getting the source code? Or of the owners building a linux version? No biggie, it works fine with Wine!

Different albums are mastered at different volume levels. That's why their volume is different. If the two tracks were of the same album it's either the artists' intent or a bad mastering engineer.
As for source codes and Linux versions: no and no, but as you say, it works very well in Wine nowadays.