Because I have the same problem, but haven't fixed it... I'll make some guesses. My vocab isn't really up to the task, either.
Your dad's sound jack inputs (?) might be dirty, or the wires behind them might be loose. The headset jacks themselves may not fit properly... can you ask him to wiggle the input thingies and see if you get any sound? If he can get any sound going with a wiggle, it's a connection problem.