When it comes to audio processing, your Mac is a Dr. Jekyll and Mr. Hyde machine. It’s a split personality PC.
On the one hand, a Mac can be a terrific audio recorder. Even Garageband is a highly capable, multi-track recording app. On the other hand, your Mac does a crummy job re-routing audio inputs to audio outputs. Without the right app, audio pass through doesn’t happen.
LineIn is a free Mac app that does just that. It lets you redirect a Mac’s audio input source to any Mac audio output source.
Simply select the audio input source from the Select menu. Then, select the audio output from the menu below that. What you get is a soft play through of audio from one source to another output.
To make the audio pass through, simply click the Pass Thru button (watch out for reverb if you’re using the built-in mic with an external speaker system).
Not bad for free, huh?
For more mature Mac users with a good memory of Mac in yesteryears, LineIn is similar to Mac OS 9’s feature, Play Input Through Output Device.