Hi!
I just thought I'd post this because it drove me crazy. I was happy happy on my Macbook Pro for a while and then my audio in SL suddenly stopped working! Why? It worked in Quicktime, it worked in iTunes? What had happened to my awsome machine?
I tried reinstalling Quicktime and Flash and everything else to no avail. Finally, after reading on several other forums about sound problems, I stumbled across a suggestion.
Close the apps that are having problems, start GarageBand, create a new song, then exit and out and try it.
And it WORKED! My Audio for SecondLife and YouTube came back!
I have no idea why this works, it's gotta be some evil driver-foo, but it works. Yay! I can hear again!
So if anyone else has that problem. Try it!