OS X Mavericks is experiencing a bug where audio goes dead system-wide. For many it seems to occur when trying to play a Flash (like YouTube) video in Chrome. For others, it just seems to occur. So far, it seems limited to new, Haswell-based 2013 MacBook Airs and MacBook Pros. So what's happening and what can you do about it?
Core Audio has always been quirky - anyone who's ever plugged in headphones only to get assaulted by the delayed fire of every mail sound in the history of mail sounds knows this - but with Mavericks the bugginess is much more specific, and annoying. When it happens, all sound is just gone. It doesn't matter if you mute or un-mute, lower or increase volume, change sources, it's just gone. Rebooting fixes it... until it happens again. There are a couple of other, quicker fixes you can try instead.
Hopefully Apple pushes out an update to fix this, and soon. In the meantime, are you experiencing the problem, and if so, any fixes working for you?