Turn the volume up using the side volume buttons
Open Control Center and raise the volume slider
Go to Settings > Sounds & Haptics and increase the Ringer and Alerts volume
Turn off Silent Mode using the Ring/Silent switch
Disable Reduce Loud Sounds in Settings > Sounds & Haptics > Headphone Safety
Turn off EQ settings in Settings > Music > EQ
Turn off Sound Check in Settings > Music
Clean the speaker grilles
Remove any case or screen protector blocking the speakers
Restart the iPhone
Update iOS to the latest version
Reset all settings in Settings > General > Transfer or Reset iPhone > Reset > Reset All Settings
Use Bluetooth or wired speakers or headphones if needed