hello David
two days ago, I had some problems related to iMic by Griffin. they hadn't any thing to do with audio. when plugging two USB cameras and the iMic into the computer, one of the cameras was never received. when iMic was unplugged, the two cameras worked fine.
some people has reported other problems related to iMic. I suspect that this iMic thing causes conflicts in the system.
this happens to you only after plugging the microphone into the iMic or, more generally, when the iMic is plugged into the computer's USB connector? have you tried plugging some line signal? have you tried what happens if you launch other audio applications?
jmb