Increase the ringer and alert volume: Settings > Sounds & Haptics > Ringer and Alerts
Turn off Change with Buttons if you want the volume to stay fixed
Make sure the alarm tone is not set to None
Choose a louder alarm sound in Clock > Alarm > Edit > Sound
Disable Sleep Schedule if it is affecting your alarm settings
Check that Silent Mode is not the issue for alarm volume
Remove or disconnect Bluetooth devices that may route audio elsewhere
Clean the speaker grille on the iPhone
Restart the iPhone
Update iOS to the latest version
Reset all settings: Settings > General > Transfer or Reset iPhone > Reset > Reset All Settings