I had the same issue. Later on, I figured out that it was the speaker been blocked by melted chocolate. It happened when I put chocolate in my pocket where my S9 was there as well. I turned off the phone, cleaned it using damped cloth with clean water. The volume is back!!!
I'll be returning my 2nd S9 for girlfriend today. In call volume barely audible. Talked to an intelligent tmo rep/tech support after signing in with a new similar and he thinks possibly it's something to do with the backup software after installation. Lasted 2 weeks on the 2nd phone and then in the middle if the call, bam, the volume dropped.
Only option is to do all new install instead of from a buckup/restore.
Same thing happened to girlfriend. 3 phones later, after talking to a knowledgable tmo tech, we figured out it was the backup. No problems since fresh install.