Here are some steps that may help.
- Open the Clock app. Add a new alarm for a minute later and select Sound > Ringtones. Choose a different built-in ringtone.
- Press the side button to turn off the display.
If the issue persists, continue with the following:
- Open the Music app > Library > Songs. Choose the ... next to a song and then choose Download.
- Open the Clock app. Add a new alarm for a minute later and select Sound > Songs > Pick a song. Choose the downloaded song.
- Press the Side button to turn off the display.
If the issue persists, continue with the following:
- Restart your iPhone.
- Update your iPhone.
- Reset all settings.
Contact Apple Support for further assistance.