Turn up the volume using the side buttons
Open Control Center and raise the volume slider
Disable Silent Mode
Turn off Do Not Disturb or Focus mode
Check the volume in the app you are using
Disconnect Bluetooth devices
Clean the speaker grills
Remove any case or screen protector blocking the speakers
Go to Settings > Sounds & Haptics and increase the ringer and alert volume
Turn off Reduce Loud Sounds in Settings > Sounds & Haptics > Headphone Safety
Turn off EQ settings in Settings > Music > EQ
Enable Late Night in Settings > Music > EQ
Restart your iPhone
Update iOS
Reset all settings if the issue continues