Try resetting all settings. This keeps your data.
Go to Settings. Tap General. Tap Transfer or Reset iPhone. Tap Reset. Choose Reset All Settings. Enter your passcode. Confirm.
This resets volume, network, and system settings to default. Many users report audio issues stop after this, especially when a glitch hit some hidden audio profile.
After the reset, turn your volume up with the side buttons, test a call, music, and a notification.