My macbook sound isn't working! Help please!

[randomuser1]

Hi! My Macbook Pro's sound suddenly stopped working earlier today. I was on YouTube and the sound was working perfectly fine, but when I went to another video the sound stopped working. I thought it may have just been the video itself so I clicked on another video. Again, no sound. I unplugged my ear buds and turned the volume up. No sound. I went to iTunes and tried to play some music. No sound. I thought that maybe my computer was too outdated so I did a software update and restarted my computer a few times. Each time I restarted the welcome sound DID appear. I don't know what's wrong! Does anybody know what's wrong and what I can do to fix it? That would be great. Thanks.

 

[GGJstudios]

What does it show in System Preferences > Sound > Output?

If you can’t hear sound from your speakers

 

[nicklad]

I've regularly had to do a...

sudo kill -9 `ps ax|grep 'coreaudio[a-z]' |awk '{print $1}'`

... in the terminal after watching YouTube videos under Lion because the entire audio subsystem fails.

A reboot works just as well, of course, but is significantly more distruptive.