My iPhone calendar stopped sounding alerts for appointments. This happened after I switched from syncing the calendar via an Exchange account to syncing via the “Gmail” account. (This really means that I was synching to Google via the Exchange protocol and now I’m syncing via the CalDAV protocol.) I made the change because I have two calendars with Google and I wanted to sync with them both. Exchange will only sync to the default/primary calendar.
So I switched to the GMail/CalDAV protocol for syncing my calendar. (I retained Exchange to sync my Contacts, because CalDAV does not sync contacts.) And my iPhone got real quiet.
When I enabled CalDAV sync, I selected sync for two of my Google calendars (via the “Calendars” button in the Calendar app). I did not select the “All GMail” option - I just selected the individual calendars I wanted. Note that the alarms I wanted were indeed in the synced calendars, but the reminders just don’t happen unless “All GMail” is selected.