If he's able to get stereo sound out of the Headphone Jack then the Audio input connections are fine.
1) The speakers mute when the Headphone Jack is plugged in, maybe that is broken.
2) Perhaps the speakers have been overloaded and blown in the past.
Either way you're going to have to open it up to find out.
OR just plug PC speakers into the Headphone Jack on the monitor... a little ugly but hey!
