Upgrading to Ubuntu 10.04 went remarkably smoothly, with the exception that audio in Flash was erratic. Sometimes it worked, sometimes it didn't. Since I use my PC as an entertainment system, this was pretty much a deal-breaker.
Googling for it got me no where. Apparently “no audio in flash” is the Ubuntu equivalent of “i have a headache” – it's a vague symptom that has multiple possible causes. Long story short, the problem was that I had both Flash 9 and Flash 10 installed. Firefox appeared to pick one at random (as far as I could tell), but audio only worked with 10. The solution was to delete Flash 9 – which I couldn't do from the package manager, because it turned out I had Flash 9 installed locally.
If this sounds familiar, type
about:plugins in to the Firefox address bar. If you see two entries for Flash, then give the following a try:
This is probably the result of some tinkering I did before upgrading to 10.04. I doubt this will be a problem with a fresh install.