Increase the ringer and alert volume in Settings > Sounds & Haptics
Turn off Change with Buttons if needed
Set the alarm sound to a louder tone in the Clock app
Choose a louder ringtone or alert tone for the alarm
Disable Sleep Focus or Do Not Disturb if they affect alerts
Make sure the alarm volume is not reduced by Bluetooth headphones or speakers
Disconnect any Bluetooth audio devices before sleeping
Clean the speaker grille
Remove any thick case covering the speaker
Place the iPhone with the speaker facing up
Enable Sound Check off in Music if it lowers playback volume
Restart the iPhone
Update iOS to the latest version
Test the alarm after adjusting volume settings
Use multiple alarms a few minutes apart
Add a second alarm with a different louder sound