I also have that happen, but with the ClickToFlash plugin and the HTML5 playback. A simple reload of the website helps me.
Though I use that feature to my advantage, as I can open new videos in the same tab without needing to load them again, and I can just browse the comments, in order to find the bastards among them. Probably because I am one of them.
I've had this happen mostly when using plugins such as ClickToFlash (long ago) or (the HTML5 player) Youtube5. Reloading or closing the page kills the video stream. Maybe it's a Flash issue, I wouldn't know
Is this a new feature on youtube? I noticed this too. If you are watching a video and then click off of it to the main page, it continues to play in a little box on the bottom right hand side. Can you confirm if this is down there when you are hearing the sound?