Canon Optura S1 Instruction Manual page 103

Sound quality may decrease when you add sound to the same scene for more
than 3 times.
You can check the picture on the LCD screen.
E
If you use [AUDIO IN] for the dubbing, you can check the audio on the
camcorder's speakers.
Set the zero set memory at the end of the scene you wish to dub audio. The
camcorder will automatically stop audio dubbing at that point.
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