Open the Settings app
Tap Sounds & Haptics
Under Ringer and Alerts, drag the volume slider left or right
Turn off Change with Buttons if you want the alarm volume to stay fixed
Use the volume buttons to adjust the ringer and alert volume if Change with Buttons is enabled
Open the Clock app and check your alarm sound in the Alarm tab
Make sure the alarm sound is not set to None
If needed, set a louder alarm tone in the alarm sound options