Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 431)

3.
Record a voice memo.
Hold down the < / > button for approx. 2 sec.
Keep holding down the button after [ Recording memo... ] appears, and
speak into the voice memo microphone (1). Each recording can be up
to approx. 30 sec.
To end the voice memo, release the button.
A [ ] icon is displayed at the top of the screen.
Caution
Voice memos cannot be added to movies or protected images.
Voice memos cannot be recorded with an external microphone.
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