Same issue has started happening to me. I suspect a recent system update to be the culprit.
It is the Google Calendar app. Version 5.7.12-150073522-release. It is synced up with my google calendar.
The phone is a "Droid Turbo 2", Android version 7.0. My provider is Verizon, not sure why that would matter.
I've experimented with the settings and discovered that I can still get vibration during an event reminder, but no audio, no matter what settings I choose. Phone is not muted, and other sounds play normally.