Start with the alarm itself
Open Clock → Alarms and check the switch, time, repeat days, label, sound, and snooze. If you use Health's Sleep Schedule, check that schedule separately.
- Enabled switch is on
- Correct AM or PM
- Tomorrow's weekday is selected
- Sound is not None
- Ringtone and Alerts level is audible
Check the surrounding iPhone settings
Use automatic date and time instead of manually changing the system clock. Apple warns that manual time changes can affect alarms. If you use StandBy, remember that its alarm haptics are disabled.
Test one change at a time
Create an alarm one minute ahead. If it works, repeat with the exact sound and configuration intended for morning. A test provides more useful evidence than repeatedly toggling unrelated settings.
If dismissing is the actual failure
An alarm can work perfectly and still fail the morning if you stop it without waking fully. That is not a volume bug. Put the phone across the room, remove snooze, or use a mission that requires a conscious action.