Prior to Mountain Lion, if I wanted to switch my audio input or output from my speakers to a headset microphone, I used a free app called SoundSource. Unfortunately, SoundSource isn’t compatible with Mountain Lion. Fortunately, there’s an equally easy way to select your audio sources on a Mac, going back at least as far as Snow Leopard.
To select your audio input and output sources, you need to use the Mac’s almighty Option key. Hold down the Option key, and click the volume icon in your menu bar. You’ll be presented with a dropdown menu, where you can select your audio source. I think I might like this better than SoundSource, because it is one less item cluttering my menu bar.