If you want an easy way to change the input or output sound source on your mac (say between the internal microphone and audio-in) in Mac OSX Snow Leopard you can simply option-click on the audio menu-extra in your menu bar.
This hint is actually from commenter Monoclast on MacUpdate.com from a review on the program SoundSource which adds a new Menu Extra to do exactly this.
[crarko adds: I tested this, and it works as described.]

