I can tell you that my two babies can be very vocal when playing. The other day I was on the phone with my sister (who has a lab and a golden) and she was alarmed at the noises she heard from my two when they were romping around the yard. I laughed and assured her they were playing. I guess people unfamiliar with yorkie noises really do think they sound alarming.
BTW, I absolutely love the set up you have! It's really quite lovely. I so wish I was able to have a doggie door to some type of outdoor enclosure so they could come and go at will. I have cats, though, who would definitely escape and we have quite a few hawks in the area, so I'd never be able to let them out alone. Now if I could just figure out a way to set up an outside enclosure that was somehow attached to the house that could be completely closed in with a roof...