a free product that only works with video on YouTube is
MP3Rocket. It's available on CNET.com (actually their sister
site DOWNLOAD.COM)

Unless its an original song on a video you made (probably not,
or you'd already have the music)the song might already be on YouTube.

If you can't find it there, I guess you could upload your AVI to
youtube, then use MP3rocket to extract the audio.