Press the Volume Up button
Open Control Center and raise the volume slider
Go to Settings > Sounds & Haptics and increase the Ringer and Alerts volume
Turn off Silent Mode
Check if Do Not Disturb or Focus mode is enabled
Disable Reduce Loud Sounds in Settings > Sounds & Haptics > Headphone Safety
Clean the speaker openings
Disconnect Bluetooth devices
Turn off Mono Audio if needed
Restart the iPhone
Update iOS
Check app-specific volume settings
Remove any case covering the speakers