In the Document Configuration window, select the existing ' Stereo In/Stereo Out' configuration. Download from here: > search for 'AU Lab'.(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.
