Increase the ringer and alert volume in Settings > Sounds & Haptics
Turn off Change with Buttons if you want the volume to stay high
Set a louder alarm tone in the Clock app
Avoid using a silent or very soft alarm sound
Turn off Sleep Focus or Do Not Disturb if they affect your alarm behavior
Make sure the alarm volume is not lowered by Bluetooth headphones or speakers
Disconnect Bluetooth devices before sleeping
Place the iPhone on a hard surface to help the sound carry
Clean the speaker grille if sound seems muffled
Update iOS if alarm volume issues continue