My husband recently got an S9 and in call volume was extremely low from day 1. Here is what we tried and it worked and increased the in call volume to an acceptable level:
Hearing ***** > Turn It Off
That solved our issue. Maybe it will help one of you, as well!
Yeah tried the damp cloth and toothbrush and turning on mono and they didn't work so tried a pin and stuck it through the tiny holes in the speaker mesh and now it's LOUD AS! thanks team!