Quick answer: Go to Settings → Sounds & Haptics and raise the Ringtone and Alerts slider. Then open the alarm in Clock and make sure Sound is not set to None. Create a one-minute test alarm to verify the result.

1. Check Ringtone and Alerts

Apple ties regular alarm loudness to the Ringtone and Alerts level in Sounds & Haptics. Changing music or video volume is not the same check.

  • Open Settings.
  • Tap Sounds & Haptics.
  • Move the Ringtone and Alerts slider while listening to the preview.
  • Turn Change with Buttons off if accidental side-button changes keep lowering it—or on if you intentionally want those buttons to control it.

2. Make sure the alarm has a sound

An alarm can be configured with Sound set to None. Open Clock, tap Alarms, edit the alarm, and choose an audible sound rather than assuming the global volume is the problem.

3. Know what headphones and StandBy change

Apple says a regular iPhone alarm plays through the built-in speakers and connected headphones. Apple also notes that StandBy disables alarm haptics, so do not depend on vibration alone in that setup.

4. Test the exact configuration

A short test catches the ordinary mistakes: wrong sound, unexpectedly low level, a missed repeat day, or the wrong alarm being edited. Ransom's Alarm Proof is built around the same principle, but includes the mission as part of the test.

Primary sources

Apple: Change alarm volume and troubleshoot quiet alarms ↗