Sony FX 6 Quick Start Manual page 35

Audio: Internal camera microphone
You can record the internal camera microphone to one or more
channels as a reference for synchronizing audio recorded to an external
recorder (but it also helps to use a slate). By default the internal
microphone level is on AUTO. The Audio Input level is the manual
recording level for the internal microphone.
manual recording level for
internal microphone
Internal microphone
on channels 3 and 4