But above all, remember they are booking a disco, and they probably know what to expect. Any special considerations would probably have been communicated to you by now, as long as you have given them the chance to do so. Just don't blast them out with techno........
Personally, I think it would be better to consider the (possibly advanced) ages of the guests and adjust the volume accordingly, rather than focus too intently on any hearing difficulties.
Have a good one!