Since posting this question, it has been revealed that the problem was with mpd — not Flash. Here is the question I posted in an attempt to find a fix.
Every time a Flash object with sound (such as a video) loads in my browser (Chromium or Firefox), it seems to break Pulseaudio. Further Flash content will play just fine, but other software, such as MPD, will not be heard. I can reset this problem by ensuring that no instance of Flash are running in my browser and running
pulseaudio -k, but the problem just comes back as soon as another Flash object initialises.
In being a little more specific, if I have sound playing (such as with MPD) and load a Flash object, sound continues to play but Flash doesn't output any sound. If I stop playing sound and then load the Flash object, Flash gets sound and seemingly takes control of Pulseaudio until
May or may not be related: When Flash doesn't get to output sound it tends to crash Chromium. I can't say if this is only when Flash gets control of Pulseaudio or not, as it doesn't always crash.
I'm running the latest version of Flash, just downloaded and installed from Adobe's site using their 32-bit deb. I have tried flashplugin-installer but the problem is the same. Running Ubuntu 10.10.
Any idea how I can fix this, or why it's happening? Does it matter that I'm running mpd as root?