problem with volume keys

hello all. I have recently been having this problem where the volume keys are frozen. When i hit the volume up or down keys (f10-f12) it displays the sound icon but underneath it has a grey no entry sign and wont let me turn the volume up or down. On the block bit where you see the volume going up or down it says it is turned all the way up but when i go onto the internet no sound is working. also
the sound on none of my applications is working. The only applications i can here is logic express 8. as what i do is mostly sound based i need to get this sorted. It is a mac OSX Version 10.4.11 thanks

This is a looooooooooooong shot, but try going Applications > Utilities > Audio midi setup. Select properties for "built in output" and make sure the format is 44100.0 Hz 2 ch 16 bit. Also, can you move the volume sliders there?

This once happened to me and I solved it that way, so maybe this also works for you.