Had the same issue, thought about getting a replacement ear piece. However, I tried cleaning like samim suggested. It improved a little. After that I startet playing around with settings in Sounds/Haptics as well as Accessibility and Music/Soundcheck Toggled everything sound related on and off, turned everything to max. Nothing. Restarted. Still no improvement.
Went through those settings again and voila! the sound is back as expected. I can't pin point which step brought the sound level back to normal, but somewhere between those steps it did happen. It was clearly a software issue. Maybe this helps someone else.
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