I've learned (luckily not the hard way) that I need to limit the current going to the solenoids to avoid damaging the sounder. There are a number of references, but I have not yet found a definitive source on what the proper current should be. Each sounder has a known resistance, so I'm going to experiment with a 12VDC Power Supply and will move a way from using the relay shield and replace it with a MOSFET. More to come.