I have time blindness, a condition which makes it hard to notice the passing of time. I couldn't find a calendar app that had alerts loud enough that I'd notice.
The iOS Clock app is loud enough, though. So I created an iOS shortcut to schedule Clock alarms based on events in my calendar: Loud Calendar Alarm.
How it works
The first thing it does is delete the alarms it created from prior day by the label “Calendar Alarm”. Then it pulls all my calendar events for the next day, subtracts 15 minutes from the time, and creates an alarm. The shortcut is scheduled to run every night at midnight.
Click Delete Always in the pop-up asking for permission.
Click Allow Always in the pop-up asking for permission to display notifications.
Click Always Allow in the pop-up asking for permission to share calendar events.
3. Schedule the shortcut
Click Automation at the bottom of the screen.
Click the +
Click Time of Day
Click the time under the blue check.
Change the time to when you want it to run (ie midnight)
Click Run Immediately
Click Notify When Run
Click Shortcut that sets alarms before calendar events, and delete them after use
The following day, confirm that you get a notification that the alarms were deleted and alarms were sent because sometimes you need to manually run the shortcut to re-give permission or even re-download the shortcut. Then it should work normally.