A Flight Warning Computer (FWC) modification is available for this....but it is not free of charge unfortunately. The volume can't be turned down by any direct engineering action so it may be better for your ops dept to find a 'less intrusive' method to get around the problem. I must admit it does lift you off your seat at 3am!
DTG
PS Sorry I can't provide a more detailed suggestion 'cos I don't think it is appropriate to discuss any procedures and methods of flight deck comms on such a public forum.