Need tool to adjust gain on audio

I'm attempting to match the level of all the audio assets in an online course. I was able to batch-adjust the gain on all the mp3 files using MP3Gain, which worked wonderfully.

However, there are a few video clips that need work. I have an avi file source for one, the rest are FLV's embedded in the timeline. (I may or may not be able to get their source files.)

Considering I don't have Premiere or Final Cut, what's a good, free Windows tool that will let me do this, or at least extract / re-import the adjusted audio?

Thanks.