Canon EOS-1D X Mark III Advanced User's Manual page 308

If the card has a fast writing speed, the indicator will not appear or the level (if
displayed) will not increase much. First, record a few test movies to see if the card
can write fast enough.
If the indicator shows that the card is full, and movie recording stops automatically,
the sound near the end of the movie may not be recorded properly.
If the card's writing speed is slow (due to fragmentation) and the indicator appears,
formatting the card may make the writing speed faster.
Audio restrictions
Note that the following restrictions apply with [Movie rec. size] compression set to
or or [
• Sound will not be recorded for approx. the last two frames.
• When you play back movies in Windows, movie images and sound may
become slightly out of synchronization.
7: Audio compression] set to [Enable].
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