Removed the alpha-numeric requirement.
It might not be a problem to your target audience but (due to general abuse of Flash on the internet) I and many others keep it on all the time.
So I used a hidden flash applet and triggered it via html events. But the flash itself was hidden since it didn't have a UI - it was just for playing sound.
Since it was hidden there was no way to click-active it for flash block people.
Maybe I could have put a message of some kind saying flash wasn't active, but it didn't seem worth the trouble. (In my case sound was nice, but not that critical.)