I just tried replicating what you did, Dave. I opened two windows, one with the weather thread and I muted Sleigh Ride. The other window was the best song on A Hard Days Night thread. I did not click on the YouTube button to "watch on YouTube.com" but just used the embedded players in the threads. I'm Happy Just To Dance With You played fine.
I have a HP PC with Vista Ultimate. I use IE8 for the internet.