Microphone and Audacity Trouble

I am using an I-Mac 5.

I just recently purchased Audacity with a sound board to record music and podcasts.

While trying to figure out how to record direct from the guitar amp into Audacity, we noticed that somehow our voices were getting caught in there. We suspect that the built-in microphone is being turned on automatically by Audacity. Is there any way to turn this off so we can prevent the microphone from interfering with the amp's sound?

It's "iMac."

Yes, you can control which mic is used for sound input by visiting the sound input preference panel in System Preferences ("Sound"->Input tab).

In Finder or in the Audacity preferences?

Finder. I don't use Audacity but AFTER changing the setting in System Prefs I would confirm that they match in Audacity's prefs (assuming it has that option).