Yeah you're right - I didn't give you all of the details. What I'm really trying to do is send an SDI signal to Wirecast for live broadcasting, and wasn't getting audio. So I was in FCP trying to capture to see if FCP would see the audio....and hit the other wall.
For anyone dealing with audio weirdness, the current (newest) version of Wirecast seems to have an issue with audio via SDI.
For troubleshooting your audio in OSX, the little mapper tool below rocks.