If you use different audio input devices with your Mac, (such as the Blue Microphones Snowball) you’ve probably had to go to System Preferences to change your selected input device. If you’re using Mac OS 10.6 Snow Leopard, however, all you have to do is hold down the Option key as you click on the volume control in the menu bar at the top of your Mac’s screen. If you have audio input devices such as microphones, USB input boxes, etc, these will show up in the drop-down list.
Time to get a microphone and kick off your karaoke career!