I cannot see the internal speaker option in the output tab in my macbook air

Download macOS High Sierra and reinstall it over what you have.

Open Safari and click on this link,

How to upgrade to macOS High Sierra - Apple Support

Go to Section 4 and click on Get macOS High Sierra,

this will redirect you to the App Store High Sierra download page

it should say Get or Download.

This will download Install macOS High Sierra.app

to your Applications folder, make a copy of it.

Double-click on it to start the installation.

Before installing make a full back up of your mac.

Just install it over what you have, your data will be fine.

Have you tried restarting your mac, if you have and if you have tried any other methods, then can you let us know.

Well done I ask you to list the methods you have used and you haven't bothered.

Do you want help or not?