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
Make sure the alarm is not set to None
Turn off Sleep Focus, Do Not Disturb, or other Focus modes
Disable Bluetooth so audio is not routed to another device
Clean the speaker grills
Remove any case that may block the speaker
Place the iPhone on a hard surface to help amplify the sound
Turn on Sound Check off for music-related settings if applicable
Update iOS to the latest version
Restart the iPhone
Test the alarm volume with a different alarm sound
Use a second alarm as backup
Set the alarm to repeat if needed