How to remove background music from lesson video?

[Query]

I know some of you video or record your lessons. What do you do if the music is too loud?

I have some videos where the music is louder than the desired voice.

Can you recommend good free or cheap software to make it easy to hear an instructor's voice over background music?

The recordings are AVI format video, but I can probably convert it to something else. (E.g., Windows Movie Maker can produce WMV files.)

The rink also creates a lot of echos. It would be nice to get rid of that too, but I suspect that is asking too much.

Thanks!