Low phone handset volume S9+ (S9 plus) Android 10

Phone: Samsung S9+ SM-G965F, e-0.16-q-20210515110657-dev-star2lte

The phone speaker audio level is very low when the phone is in normal handset mode (i.e. no earphones, not speaker mode).

This is audio from the top speaker, even on maximum volume, close to the ear it is quiet.

Is this a general problem with this phone and ROM, or mine alone?
How can it be fixed?

(Note if you notice this duplicates part of Audio Problems: Galaxy s9+ e0.16 android 10, it is because I’m splitting that into one post per problem for better visibility and focus)