It's very important i can find out tonight!
This happens a lot on my MacBook but i usually restart and that works i'm going on holiday tomorrow as well. But the flickr urls are bellow on what's happened but basically the sound stops and when I press sound up it just beeps and shows what's on the images the difference is one has a lock and usually has the sound bars full and the other one has it on "Full" but the circle is white and you can't move it.
so thank you here is the image urls
Main One (Locked circle)
Top Bar One