MacBook Pro not outputting audio
In the first instance, please repair disk permissions. If that makes no difference please try starting your machine in Safe Mode before restarting (back to normal mode) and seeing if that made any difference.

If neither of these work I would reinstall the OS over the top at this stage.

As to the cause? I think the Quicktime system plugin has incorrect permissions, is damaged or just plain isn't where it should be. Let us know how you get on!