Audio In allows me to listen through my G5 speakers as I edit recordings on my minidisc recorder (MZR-5ST, which has optical in/out ports). I run an optical line out from my minidisc into my G5's optical in port.

Before, I had to use headphones plugged into my minidisc, and they never seem to be around when I need them! I do a lot of recording for language learning on a minidisc recorder.

There are other applications that I have which do this, by the way, like Amadeus II. However, Audio In is nice and small and easier to set up and use for play through. And it's free!

Thanks!!