Do I have a microphone problem on my 27" ACD???

I finally just got my new 3.33 6 core mac pro with a 27" ACD and have been doing some preliminary testing to make sure everything is behaving and working as expected. Up till I had this machine I've been using Tiger 10.4.11 so jumping "up" (?) to Snow Leopard 10.6.4 takes a little getting used to.

One of my tests involves the Sound Preferences, specifically Input. I am not sure the microphone is working on the ACD. If I set the sound input to "Display Audio" - type USB (also tried all the others too) then I would expect to see some activity on the Input level display. I don't see anything no matter what the Input volume setting is. I can bang on the ACD near the microphone, clap my hands, shout, whatever and see no input level activity. On the other hand I also currently have the Sound preferences up on my nearby Tiger machine and it's microphone can hear me typing this as indicated by the Input lever meter on that machine.

So I am wondering if the microphone working on my new 27" ACD or some additional "magic" needs to be performed in Snow Leopard that I am not aware of.

In case anyone should ask, yes, I do have both the mDP and USB cables plugged in. And I got the Input volume turned all the way up and still nothing.

Can anybody help?

After spending some time talking to Apple support about my microphone problem, and after going through their suggested (probably scripted?) steps, they couldn't get my microphone to work. So they set up an appointment with the local Apple store.

I lugged both my ACD and Mac Pro (just in case) to the Apple store (my major exercised for the day, yuk). They hooked my monitor up to a convenient Macbook Pro lying around and sure enough confirmed that the microphone was not working in my ACD!

So they exchanged it for another new ACD. Lugged it (and the Mac Pro of course) back home (yet more exercise), hooked it up, and the microphone works with this one.

Of course I needed to again verify the new ACD didn't have any bleed through or dead pixels. Fortunately it looks good. So that's the end of this "adventure".