I have the very same problem. Randomly, the phone is on mute, even if all the volumes are maxed, DND is off, and I didn't touch the phone.
If I reset the phone (10 secs power button), it's ok, but just until it happen again. It's been some months since I'm experiencing this problem.
You don't know when and if it'll ring. My "test" is the charging cable. If it doesn't make the charging sound, it's on "mute"