Not sure when it started exactly, but for the past few days the volume on my MBA is not working. What I press the volume buttons on the keyboard, it shows the volume adjusting on the screen like normal, but the accompanying noise doesn't happen. Sound DOES work on Messages though, which is weird, and when I open sound prefs and click the sounds there they work too, but youtube videos, music in iTunes, d/led videos, etc give me nothing. Help!