How to disable ambient noise reduction? It used to be in that one could disable the ambient noise reduction in the sound preferences.  But I can't seem to find that option anymore, which means when I record sound with the built-in mic I have to accept the warbling effect.

According to Help, the option should be there so long as there isn't a 4-channel built-in microphone format chosen in Audio MIDI setup.  But my internal microphone is a 2-channel according to Audio MIDI Setup.  

How can I get the check box to appear so that I can cancel the noise reduction effect? I have a Late 2013 27" iMac
This model of iMac (Late 2013, 27") has dual internal microphones, which were designed to offer better noise cancellation. This is most likely why the Ambient Noise Reduction checkbox is missing.