Many Mac users have several input and output devices for sound. For example, I have a mic, external speakers and a set of headphones connected to my MacBook Pro pretty much all the time. I also use Soundflower for capturing audio from my Mac. I can make any one of them active via the sound System Preference. Or, I could use this trick, which is much faster.
Just Option-click on the volume icon in the Menu Bar and presto! A list of available input and output devices appears. Just click the one you want and you're all set. Nice and fast.