Ring tone super-quiet on Galaxy S9+
As suggested I am posting how it was "resolved".

Went to Samsung and it turns out that the bottom speaker isn't working.

That's why it is so weak. The fact that it works from time to time probably means that the problem corrosion or something similar.

This also explains two other problems I have had that I thought were unconnected:
The "beep" when plugged in (as I mentioned in the question)
The fact than when I am speaker lift at to my ear I can no longer hear the call. I guess that the sound then comes out only the bottom speaker.

Edit:
It turns out that it rings when flat on a surface only. It seems that when it is vertical it tries to ring from the top and bottom whereas when flat it only rings from the top.