Is a bug I think. I have mentioned this a few times already in the forums, and it has happened to me too. As pezzman2k said, just open Audio MIDI Setup's audio output format is set to 44100.0 Hz. If you own a copy of GarageBand, you can open and close it and that will also fix the issue.
I should also note that the problem is not only for Safari. I have seen it in a couple of games too and in Virtual PC.