#554 No net game microphone in OS X (error message included)

[Barcode, the 711th]

OS X Snow Leopard
Macbook Pro, Summer 2009

Error found in logs (not sure if at startup or initialization of net game, but it immediately precedes the packet loss errors that typically follow netgames):
[0x0-0x23b23b].org.source.bungie.marathoninfinity[3268] failed to set AU input device

The computer has a built-in, functioning microphone (which is chosen as the default audio input device in the Sound section of the System Preferences), so I'm not sure what the deal might be - or if there's any way to force Marathon to see this microphone.

(Error description: I press the microphone button, I send no audio. People can see that I'm TRYING to transmit audio, at least - they see my name and the transmitting thing in the bottom right corner.)

[Barcode, the 711th]

I forgot to mention:
-I'm using the all-in-one package for Marathon Infinity.