I also had the same issue on an XR after updating to 15.3.1.
I looked up the on-line advice and carefully cleaned out the speaker holes, ear speaker hole etc. I called Apple customer support to see what the advice was and they said it was most likely a hardware fault and the cost of mail in repair was £380 ($500)!
I booked a Genius Bar appointment at the nearest Apple store to double-check. The ran audio tests and determined that there was a problem with both the ear speaker and the mic, cleaned it properly and Hey Presto!, it now works fine again. Well worth the journey to fix this!
I have a theory that the update in someway changed the sensitivity of the software feedback from the mic/speaker which reduces the volume, hence why the "fault" appeared immediately after the software update.