A simple solution: launch Garage Band, then quit. Garage Band regains control of the sound card for Safari and other web browsers' use.
[kirkmc adds: I have a feeling that what's happening is the same thing described in this hint. I had this problem in the past not only with Flash, but also with system sounds; I always used the Audio Midi Setup solution, but since I've had a Mac Pro, I haven't had the problem. (I assume the sound is managed differently, since there are multiple inputs and outputs.) But this solution (launching and quitting GarageBand) is probably simpler for most users than fiddling with the settings in Audio Midi Setup.]

