What firmware version do you have? Settings/System/About (version+build)
What YT app version do you have? Highlight YT app/Options (*) (version+build)
From checking, it appears that YT app version 2.20.93005147 (for non-4K/HDR models) does not have the preview sound control functionality, so if you have that version, that may be why.
The 3920 is a 4K/HDR model, and should have 2.21.94005088 (for 4K/HDR models) which does have the preview sound control functionality.
You should contact Roku/Google about this YT version issue for the 3920.