he must have set up an alarm for something and it is repeating.

You should be able to find the windows sound & turn it off in the control panel -
sounds and audio devices - sounds tab - then look for alarm in the list or listen to the sounds to find the one that's the same.

I don't like any of the sounds so I just select No sounds -- speakers will still work fine for music & videos etc - just none of the windows sounds will happen.