I suspect a significant difference in voltage was created between the structure's electrical system "ground" and the cable system "ground" due to local lightning strikes. The excess voltage followed all possible paths to earth and unfortunatly some of it was thru the A/V equipment.

How old is the church's electrical Service? Specifically the grounding system.

What is the soil like? Red clay?

As far as fluctuating voltage levels, a good quality UPS should minimize the dropping out of the electronics due to low voltage.

A multi-level approach to prevent this issue from happening again can be as follows:

1) Keep all electrical and electronics referenced to the same level. It is not the voltage that did you in, it was the DIFFERENCE of voltage that did you in. Create a quality low impeadance earthing system. TIE EVERTHING TO IT! That means the electrical service, phone service, cable service, structual steel, in ground pool and other water features, and all other metal piped services. Create a good ground reference in each room with an electronics rack. Try 6 inch wide heavy duty copper flashing as a ground conductor. Make all of the bonding conductors as short as possible and as wide as possible.

2) Multiple levels of surge suppresion. Use a surge suppression system at the main panel. Followed by one at the sub panels that feed the A/V systems. Followed by local surge suppressors at individual loads if necessary. Install surge suppression systems on the Phone lines where they enter the building. Do the same for the cable lines WHERE THEY ENTER THE BUILDING. All of the surge suppressors should be bonded to the same grounding system.

3) Protect against brownouts and short term power outages by using QUALITY Uninteruptable Power Supplies (UPS). Quality means units that isolate the output power from the input power.

I am assuming that the church was built or updated with properly grounded outlets everywhere and all of the wiring is in steel conduit.

Does the church have a lightning protection system? If so, is it connected the rest of the grounding system?