Go to Settings/System Settings/Volume and make sure that hasn’t been accidentally switched from High down to Normal.
Another thing that can make the volume lower is if the geographical zone gets set to Europe, due to EU regulations to prevent deafness. You’ll go back to defaul settings under Settings/System Settings–which means you’ll have to put back in any of your own settings–if you Reset Factory Settings. But if you want to try that you can make sure the zone is USA or Rest of the World instead of Europe.
