ETC Congo Family v6.4.0 - Manual (Page 244)

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Sound Settings (6.3)
The volume of the internal speaker is set by holding SETUP and moving the level Wheel.
You will see the result in the value next to the loudspeaker symbol in the top right corner of
the monitor screens.
This will take effect immediately and is NOT related to different play files.
You can also right-click on the speaker to open a context menu for setting sound levels (6.3).