If you want the sound to be as good as it can be then any of the Relay or XDV systems would work great.  Just depends on the number of channels you want to use and the distances you need to hit.

Just plug the line outputs from any mixer into the beltpack transmitters and plug the output of the receivers into a mic level input on the speaker systems.  You may need to add 15 dB pads to the outputs of the mixer to keep it from overdriving the  inputs of the transmitters (although they will handle about 6 volts peak ac at their inputs).