I'm wondering if the two distant rooms weren't on opposing legs of the 240vAC feed coming into that section of the building. (Assuming we're talking about US power.) In that case, both outlets will test fine, but the "120v" in one room is "anti-phase" with the "120v" in the other room, and there's effectively a 240vAC difference between their hot leads. (For the prudists, this is not "really" 2-phase, it is both legs of the center-tapped secondary coming from out at the service pole transformer.) If the neutrals and grounds were indeed the same at the two outlets, and they are wired correctly, then a defective or poorly-constructed device may still cause serious issues. I had this very thing happen a few weeks ago, and it destroyed a (cheaply made) musical instrument amplifier. So I set out to determine once and for all, how this could happen.
First, lets expand on the issue of "faulty residential ground wiring." What does that mean exactly? Well, for typical devices, "neutral" and "ground" work like this: http://www.allaboutcircuits.com/vol_1/chpt_3/8.html
Here is another link to understanding "neutral" and "ground" in residential power: http://forum.allaboutcircuits.com/threads/confused-about-ground-in-ac-circuits.76003/
Now those are for standard appliances, and always assume the "hot" feeds are identical. Well, in the real world the feeds often are not, because the electricity coming into the building has more than one "hot" feed. (It typically must, for powering 220vAC appliances.) This is how an electric water heater, electric clothes dryer, or air-conditioner get 220v... from two 120v "hot" feeds which are out-of-phase with respect to each other.
If we were to connect a dual-channel oscilloscope (using x10 probes!) directly to both "hot" feeds at the panel, we'd see:
These two 120v "phases" are routed to the left and right sides of the power distribution panel respectively, and supply 120v to circuits on the left, and 120v circuits on the right. When the electrician wires room circuitry to the panel, (s)he distributes those branches to both sides evenly. Kitchen on the left, bathroom on the right, etc. This must be done to balance the load presented to the power company.
For a 220v appliance, it is simply connected to both “hots.” Since out-of-phase AC “adds”, that appliance “sees”:
So what would happen if we had a poorly-isolated device on each "hot" leg and started poking and prodding around between them? For these images, right-click and "Open in a New Window" to see the full-resolution versions.
Here we have an LTSpice simulation of two pieces of poorly-designed electrical mains equipment which do not employ electrical isolation. This is an important point, because most electronic appliances DO use transformers to step-down the line voltage into something more manageable by the device. Transformers are very important for safety, as there is no electrical connection whatsoever between the primary and secondary sides, so this issue I'm about to describe will not occur. Some devices which may not provide electrical isolation are high-power audio amplifiers, certain switch-mode (buck, boost, and other topology) power supplies, very cheaply-made devices, etc.
In general, the vast majority of devices are designed well, and this issue should never occur. In the real world however, I've personally seen this damage three pieces of equipment in three decades (and I've been shocked just as many.) It usually isn't much of an issue, partially because appliance cords are intentionally made to be short enough that two appliances from different "hot" legs will never touch, and the vast majority of the populace wouldn't be trying that anyways. But running any length of wiring longer than six feet can impose some serious issues, as many musicians have found out the hard way!
So say Device A and Device B are cheap 3000W power amplifiers for large performances. They don't use mains transformers, because a 3KVA transformer would be impractically large and heavy. Instead they are a low-cost, new "Class D" type. To get 3000W out of one, it just rectifies the mains voltage directly to 155vDC, filters it with some caps, and modulates that to drive the speakers. To put 3000W into an 8-ohm load, P=EI --> I=E/R --> P=E(E/R) --> P=E^2/R --> E^2=PR --> E = SQRT(3000*8) --> E = 155vDC. So between +v1 and -v1, we see:
Which looks like a DC voltage, and it is – with respect to those two terminals. But what do we see if we measure between the neutral and these?
Compared to ground or neutral, these lines peak at 310v difference! The reason why we "see" 155v DC between them, is that these are in-phase with each other, and the net difference between them is a constant (hence DC) 155v. So even though their absolute voltages are very high, their relative voltages are a fairly constant 155vDC.
The same holds true for Device B, it also produces 155vDC, and it's terminals also swing an absolute range of 310v, just like Device A... with one exception. Since it is being fed from the other "hot" lead, it's phase is 180 degrees out, which can be seen from the ripple current differences:
Now on the DC side of things, this means nothing. But on the AC side, this means a lot, since opposite AC polarities add.
Therefore, introducing any kind of electrical conductor between these two circuits in any way, is going to have bad consequences, because they are out-of-phase. But just how bad? Lets look at what is going on across the “accidental short” resistor R4:
Green is the 155vDC output of Device A, red is the voltage across R4. It is “seeing” 120vAC, however since it is such a small value, the voltage drops to around 90vAC. If we look at how much power is going into R4,
With a peak of nearly 2.2kW, it is going to spark and burn!
Now I have no idea if one or more of your devices are at fault, or if it's this HotA/HotB issue. What I'd do is the following:
Use an electrical outlet tester (such as https://www.google.com/shopping/product/12841723546621710763) on ALL (each and every one) of any outlets that can possibly be used by this equipment as @alex.forencich suggested. All of them must show correct wiring. If any one is not wired correctly, call in a certified electrician to fix it.
If all of the outlets are good, then it could be this HotA/HotB issue. You could run an extension cord from each location, and measure AC volts between the two smaller “hot” pins. It should read about zero if on the same feed (good) and 240vAC if on different feeds (bad.) 220vAC is quite hazardous, so if you don't feel comfortable and knowledgeable in testing it, leave it for a certified electrician. Discuss what you're trying to do with them, and they will likely swap around some circuits in the distribution panel to put those branches all on the same “side” of the feed.
Now, that all said... the church may not have 2-phase 220v power, especially if it is a larger one. It may be 3-phase, 208v. In other countries, it can vary even more. If that's the case, definitely call in an electrician. Ask to see their certification, you want this to be fixed properly the first time.
