You mean three transmitters (each having three channels) and six receivers, correct? Three transmitters and receivers, one pair in each room, and three additional receivers: back, middle, and reception?Good idea on the flashing light, I like that. No, will not ever need any more than three sending locations and three receiving locations. The only receiving station would be behind a counter in the "front", where clients could not see it.
I'm looking at latching remote modules on eBay but they seem a bit pricey to me. I'm thinking we can get away with a less expensive non-latching transmitter/receiver pair and add a toggle flip flop which will turn the momentary signal into a latching one. By using the non-latching type, we can use a 4.5-5.5VDC power supply wall wart and avoid having to add a voltage regulator for the sound module. We can then use LED's that have a flashing circuit built in.
I actually have several of the momentary remote pairs left over from another project, so I'll design a circuit and test it then report back. I may need a few days to do so, but I'll let you know as soon as I have some results. Others may have some better ideas, so I'll continue to keep an eye out.