lost audio on macbook

I've lost all audio on internet and itunes on my macbook. But I can hear sound effects when i increase or decrease the volume on the computer.

Can anyone help, i'm not too technical?

Do you have external speakers hooked up to it? If so, where do you hear the sound effects from? (unplug the speakers and see if you still hear them)

Do you remember doing anything, like installing something, between the last time it worked and the first time it didn't?