Press the Volume Up button on the side of the iPhone
Open Control Center and drag the volume slider up
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
Disconnect Bluetooth devices that may be playing audio
Check the volume inside the app you are using
Clean the speaker grille
Restart the iPhone
Update iOS to the latest version
Reset all settings if the volume still seems low