Sometimes, if minecraft runs out of memory, OpenAL will crash, causing minecraft to enter silent mode. Happened to us a few times when testing new sound code.
Restarting minecraft should fix it. If you run the game from commandline, then you should be able to see what happens in the command prompt.
Yeah, I'm going to double check via the Terminal, something isn't right here.
EDIT: I think the lwjgl needs to be updated. I'll report back once I get the results.