I will probably find that I am speaking too soon but so it goes...

This morning when I turned on the computer I noticed thatit didn't make its usual sound. Later I discovered that various programs that normally beep or sound or whump are not audible either. Now I find that the CD Rom doesn't play audibly either (It says it is playing but it is silent).

I've checked the Volume level and the Speaker levels and I've checked whether it's plugged in. All normal.

So what is the problem?