iPhone alarm not going off? Check the sound and schedule

In Clock, open Alarms and edit the alarm that failed. Under Sound, pick a built-in tone and save the alarm. Apple says Silent mode and Do Not Disturb still allow Clock alarms to sound. If you use StandBy, choose a tone you can hear because alarm vibration is disabled in StandBy.

Open Settings → Sounds & Haptics. If you see Alarms and Timers, turn off Match Ringtone Volume there and set its own volume, or turn it on to use your ringtone volume. On versions with a Ringtone and Alerts slider, adjust that slider. The volume you hear while playing a video is a poor guide to how loud the alarm will be.

Check the switch beside the alarm time and look at Repeat. A weekday-only alarm will skip the weekend. Check AM or PM if you use a clock that shows both. If the alarm is under Sleep in Health, edit the next wake-up time under Your Schedule. Check that Alarm is enabled and adjust its sound and volume there. For a change that should repeat, edit the active days and alarm under Full Schedule & Options.

Set a separate Clock alarm for a few minutes from now with a built-in tone. Lock the phone and leave it as you normally do overnight, including StandBy if you use it. If the test rings, recheck the specific overnight alarm and its schedule. If the test stays silent with an audible tone and volume selected, note whether the alert appeared on screen and contact Apple Support. Use a separate alarm device for your next important wake-up until you can reproduce a successful test.