Once opened up, just click on 'Play Through'. ) It works perfectly on my Powerbook (OS El Capitan).
(You can download MTCoreAudio for Mac for free at. I tried several things and was getting pretty frustrated until I discovered an old 2010 article by Adam Dachis about using AudioMonitor, part of MTCoreAudio’s developer package for Apple products. If necessary, adjust the system's overall input and output volume settings in System Preferences → Sound → Input. In the new 'Untitled' window that opens, ensure the icon at the bottom says ' Audio Engine Running' (or click to toggle if it says 'Audio Engine Stopped'). (If the Document Configuration window is not already open, click File → New to open it.) Then click the Create Document button in the bottom right corner. In the Document Configuration window, select the existing ' Stereo In/Stereo Out' configuration. (If those links die, it can also be downloaded from Apple Developer Tools, which may require an Apple Developer account.)
(Well, nearly in real-time for me there is a slight delay from input to output.) As originally suggested by qu1j0t3, Apple's 'AU Lab' provides the ability to listen to the audio input in real-time.
