New iMac: how can I get the internal mic to turn off?

Question

I always have Facetime cameras and internal mics disabled for privacy. Cameras are easy to block, but the Sound pane doesn't allow me to disable the mic, only turn the input down to the bottom. Supposedly, its off, right?
Poking around in Prefs, I saw that my supposedly "disabled" microphone showed a response in the Dictation & Speech pane VU meter when I would snap my fingers around the bezel. Even worse, it showed a response to conversation in another room, the refrigerator starting up in the kitchen 25 feet away, and even an increase in ambient noise. Going back to the Sound prefs, the mic input is still turned "down" and doesn't show a response when I snap my fingers.
So... how is it that this supposedly defeated mic is working normally, and in fact exceeding expectations, despite being effectively turned "off" in the Sound pane?
Basically, this iMac is a very expensive bug sitting on my desk. Is there a way to properly disable the mic in the system? I can't even find the thing to block it physically, like with bezel cameras. There is no place for an external mic to plug in either, which would physically bypass any internal mic on my previous Macs.

Answer 1

Interested parties may want to know...
I went to the Apple Store two weeks ago for a replacement Airport Express. While I was waiting for it, I struck up a conversation with one of the store staff, and brought up the mic issue. We spent a good ten minutes going through the few ways of securing the mic, and we determined that every iMac made has the ability to continue listening evne though dictation is shut off. Unlike old Macs, you can't disable the mic in the Sound prefs. You can't turn off listening in the Dictation prefs - even though turning off "dictation" means the dictation part of the OS stops listening. The only way to definintely shut off the "listening" part is to keep the Audio Devices panel open, select Built-in Microphone, and make sure the master mute button is checked. Also, you have to leave the Dictation & Speech pane open in Preferences to watch the mic display.
Even so, going to any other preference panel and then back to Dictation & Speech means that the master mute will uncheck itelf and the mic goes back to listening mode.
The staff member was quite frustrated by this and said it appears the only way to stop the iMac from listening with any certainty would be to block the mic, just like millions of users block the Facetime camera with a piece of tape.
I suppose I should admit defeat at this point, and maybe I should have long ago recognized that no matter what the system tells us, we shouldn't trust it. Simply clicking on a radio button to disable what in all actuality is really a room bug shouldn't have us letting out a sigh of relief and saying "Wow I'm glad I was able to stop that security issue by trusting the machine to let me know when it was listening."
In another year my Apple Care will run out. Then I'm going to take this iMac to my local authorized service center to have a hybrid drive installed and the internal memory maxed out. While they're in there, I'll have the mic physically removed.