this isn't too difficult to do, there are many ways of doing it.
The way i would do it is with airport expresses and small hi-fi/speaker systems. If you put an airport express into each room they are all capable of receiving a steam from the imac. You then plug the airport express into a hi-fi which will give you control over the volume in each room.
In the waiting room you would also have the same setup and also plug the tv audio output into the hi-fi, then someone can choose the tv or the airport express as the source.
Airports only technically stream from itunes but you can buy a cheep app called airfoil which will stream music from any program i.e pandora.