Canon EOS-1D X Mark III Advanced User's Manual page 286

Attenuator
Automatically suppresses sound distortion caused by loud noises. Even if [Sound rec.] is
set to [Auto] or [Manual] for recording, sound distortion may still result if there is a very loud
sound. In this case, setting it to [Enable] is recommended.
Microphone
Normally, monaural sound is recorded with the built-in movie microphone.
Stereo sound can be recorded by connecting an external microphone with a 3.5 mm
mini-jack to the camera's external microphone IN terminal (
use instead. Using an external microphone such as Directional Stereo Microphone
DM-E1 (sold separately) is recommended.
Line input
Direct input is supported for line-out stereo sound from mixers or other devices. Audio
from a 3.5 mm mini-jack connected to the camera's line input terminal (
with video to record movies with stereo sound. The standard input level is –8 dBV.
Adjust the audio recording level to suit the line-out level.
Headphones
By connecting commercially available headphones with a 3.5 mm mini-jack to the
headphone terminal (
listen in stereo to audio from an external stereo microphone. To adjust headphone
volume, press the <
( ).
You can also use headphones during movie playback.
), you can listen to the sound as movies are recorded. You can
> button, select <
286
), which the camera will
>, then turn the <
) is combined
> dial for adjustment