So far as it has ever been explained to me it is not possible to do volume leveling with video files. I don't remember exactly why, but Matt has explained it on more than one occasion if you're in the mood for the forum search game.
One option that may be acceptable to you is to use a VST compressor plugin. This will squash the dynamics of your music though. I believe this is similar to what TV and DVD player manufacturers use for their "night mode" or "keep commercials quiet feature", so the effect it has on your music might be something you're used to already. It's worth a shot if nobody has a better suggestion at any rate.
There are a lot of freeware VST compressor plugins available out there. I don't personally know of any that were more or less designed for this purpose, but you can check out the ones here:
http://varietyofsound.wordpress.com/vst-effects/ These are high quality plugins, but they are designed with audio mixing in mind rather than consumer end use, so you could wind up with some strange sounding results. Or they could work wonderfully.
Hope this helps a bit. -JB