I've just received the following link from my health centre about Dexcom Receivers : https://mhra-gov.filecamp.com/s/d/G6YkgDmoIrZhVqrw
This is not about the smartphone apps, which I use, but the stand-alone Receivers. Please check it out, if you have one.
Looks like a speaker issue with the alarms.
Though to be fair they can still vibrate..
Could be an OS issue in settings that drive the speaker. At a guess...