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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. 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.)
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As originally suggested by qu1j0t3, Apple's "AU Lab" provides the ability to listen to the audio input in real-time.
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