Increase the ringer and alert volume in Settings > Sounds & Haptics
Turn off Change with Buttons if you want the volume to stay high
Set the alarm sound to a louder tone in the Clock app
Avoid using silent or very soft alarm tones
Make sure Do Not Disturb and Sleep Focus are not muting the alarm sound
Disable Attention Aware Features in Settings > Face ID & Passcode
Turn off Bluetooth so the alarm does not play through connected headphones or speakers
Place the iPhone on a hard surface to make the alarm easier to hear
Remove any thick case that may muffle the sound
Clean the speaker grille
Update iPhone to the latest iOS version
Restart the iPhone if the volume seems unusually low