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.
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.
