Canon EOS-1D X Mark III Advanced User's Manual page 436

3.
In the single-image display, tap <
4.
Press < > to play back the movie.
(1) Speaker
The movie will start playing back.
You can pause playback and display the movie playback panel by
pressing <
Turn the <
>. Press it again to resume the playback.
> dial to adjust the volume (even during playback).
436
> or press < >.