Turning off built-in mic on iBook?

[Larry Dersch]

This is a real problem for me.  I know there must be a simple solution. 
  I'm new to X (10.2.8), having just acquired an iBook...oh, is my 
Lombard jealous!  I can't get the built-in mic to turn off.  I can get 
it to turn down, but I still hear room sounds through my speakers.  
I've checked Speech Recognition to be sure it's turned off (it is).  
Not many options in the Sound preferences either.  What's up?

Thanks,

Larry Dersch