Ok, that makes sense. Thanks for getting back to me.

Nothing to do with this conversation, but I actually ended up in the company of two lawyers who specialize in music copyright earlier today. Seizing the opportunity, I queried them about this very topic. To give you the short answer (they were lawyers, after all), if you don't have the original songwriter/composer's permission, you could find yourself in a bit of trouble - potentially YouTube could shut down your channel, or, if things turned really nasty, you could find yourself in court (even if you didn't monetize the video). Hope this helps anyone reading this. I'll certainly be staying well away from cover music!