Microphone off??

[Dave]

Am I being totally stupid here, or is there no way to turn off the 
internal mic on my iBook 700? I only noticed it was on when I 
accidentally tapped the back of the computer with an ethernet cable 
just now; system prefs just gives me the option to mute the output, or 
turn the input down low...

On a possibly related issue, my speakers have started to whine horribly 
when set to maximum or just one down volume; I usually have the 'Book 
plugged into my stereo, but....

Should I be worried??

Thanks!