Canon EOS-1D X Mark III Advanced User's Manual page 849

Audio compression
Sets audio compression for movie recording. [Disable] allows higher audio quality than
when audio is compressed, but file sizes are larger.
ON: Enable
OFF: Disable
Caution
Editing movie files recorded with [Disable] and then saving them with compression
will also compress the audio.
Audio is compressed even if [Disable] is selected when [Movie rec quality] is set
to (NTSC) or (PAL).
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