I will say that I completely disagree with what was said above. It does look like you have decent speakers. The lack of volume control being connected does indicate that they were trying to keep little hands off the volume in the zones. NOT hooking them up properly when they left is just bad form, but easy for a handy person to fix (which it sounds like you are). Whole house audio systems, that are easy to use, really don't take away from a home's value at all.
Check out the volume controls that were in use. Please let us know the make/model. Double check that they are of decent design.
Be aware that using a surround receiver for a whole house audio distribution hub is not at all what they were designed for. They really are designed to provide audio to a single room.
THAT SAID...
If you want to use your Yamaha, I would suggest pulling all extraneous transformers out, and properly wiring the speakers, as stereo, into the volume controls. Check each zone one by one by one with the Yamaha to ensure that audio is playing out of the speakers properly.
Then get a impedance matching speaker selector. This will allow you to run all your zones at the same time and have the volume controls work properly. All zones will be on when the Yamaha is on, but you can turn them off with the speaker selector, or by going into a room and just turning the volume down. The speaker selector is very inexpensive and will give you whole house audio on the cheap.
I do like the idea of a 8 channel or larger amplifier designed for whole house audio as that will give you the most reliable sound possible. I use Rotel RMB-1048 amplifiers in my home and they are now about 15+ years old. Every one of them works perfectly. So, you know.... reliable.
You have a ton of actual options when it comes to whole house audio (WHA) and it runs the gamut in terms of cost. But, at the very least, you can go through the steps of getting everything up and running. If you don't feel confident in your ability to do so yourself, you can contact a local pro who can help you out as well.