Hi JazzCat,
I posted a couple of links for you about Copyright law and Fair Use in the world of music–Section 107 of federal law (more specifically H.R. Rep. No. 94-1476, and The United States Copyright Law) allows for fair use of copyright material and it lists 4 general criteria that it mentions to what constitutes fair use. I have bought/shared some backing tracks from Karaoke with folks on my youtube channel—I’m don’t selling them or anything and this is perfectly legal under federal law. Youtube has never given me any problems either. Some music companies can decide to put a “block” on whether the track can be seen in other countries or not and I’ve only had that happen to one video that the music company blocked it from being viewed only in Germany. Other than that, I’ve never had any problems.
If I was going to make a CD, for example, I would have to purchase a licensing/user agreement where I would have to basically purchase the rights from the music company.

http://copyright.musiclibraryassoc.org/Resources/CopyrightAndFairUse
http://www.pdinfo.com/copyright-law/music-and-fair-use.php

Good points Jazz Cat, thanks for sharing. It can vary from song-to-song, music company-to-music company, etc.. It’s always a good rule of thumb to follow, when in doubt then check it out so that way we cover our tracks. The ones I share with on my youtube channel are all generic tracks and are widely available to anyone, I’ve never had any problems–it’s been a cool way to connect with other people on youtube.