A damaged Flex Cable #3 or damaged Connector #3 on Logic board may still have partial function like you are experiencing. The individual traces or components that are damaged, are broken, the ones that are not damaged still can work.
First step is to Replace Flex Cable #3, when you are there you can check with a voltmeter for conductance across the earspeaker. Also look for damage to the #3 connector on the Logic Board.
If still problems could be a damaged #3 connector on Logic Board as the repairman told you.