I have no recollection of what actually made the engines stay in sync. I have a feeling it was rpm-related though, because they were held to within 3% PRM of the 'master' engine to prevent problems if you feathered one or had an overspeed.
Definitely, engines with the PRTs were a more subdued sound compared to those without. That's for the 3350s; 2800s and les did not have PRTs.
I can't really remember much difference on the DC-6 with sync on or off. A good engineer could sync 'em just as good.