I recently downgraded from Ubuntu 11.10 to 10.10 on my laptop (to get the old desktop environment with panel applets back), and now I have discovered that plugging headphones into my headphone jack doesn't automatically mute my speakers; I get some out of both my speakers and my headphones. In fact, I can't find any options to enable this. I have never had any problems like this before; not on 11.10 on the same machine or 10.10 on my old laptop, so there must be some option I'm missing to enable this. How do I do it?