MacBook Air issue: Keyboard and in built mic failing The keys 9,o,l and . on the keyboard of my MacBook Air have stopped functioning all of a sudden. Instead of the intended characters, random characters from the same row as the faulty keys appear. 

Also, the built-in mic is failing.

What could be the problem?

EDIT: The keyboard and mic problem started to appear after a defective screen panel was replaced a few days ago. I wonder how that could have caused the side effects I'm experiencing.
Well to me that sounds like hardware failure.  I'd suggest that you just take it in to your local Apple Store or AASP; I don't see why they WOULDN'T replace the necessary parts (I don't know the MBA assembly well) if it's under warranty.