Increase the volume using the side volume buttons
Open Control Center and raise the media volume slider
Turn off Low Power Mode
Disable Reduce Loud Sounds in Settings > Sounds & Haptics > Headphone Safety
Turn off EQ settings in Settings > Music > EQ
Try the Late Night EQ setting in Settings > Music > EQ
Turn off Sound Check in Settings > Music
Clean the speaker grills
Remove the phone case
Make sure Bluetooth is not connected to a quieter device
Restart the iPhone
Update iOS
Use a music app with its own volume control and increase it
Check if the app has a volume limiter and disable it
Reset all settings if the issue continues