Not sure what your ‘old days’ system looked like, but even radio stations through the air waves were not really synced.  You and neighbors system could still be off by a second or so do to time it takes the radio signal to reach your individual systems and processing, not to mention speed of sound and all that.

But anyway, to the actual issue, I’m not aware of any great way to accomplish this.  If wired connections are not an option. then each households system needs to have a way to delay audio playback so that can be buffered to play in sync with other households.   Sonos can add a bit of delay, but perhaps not enough range for your needs.  The other side of this is the source. it obviously would need to be the same source...either an actual radio station with skip functions, or a ‘local’ source you’ve setup to feed the neighborhood.  Ideally, if you had a local source that could feed neighborhood homes and control delay for each feed at the source, then you would be able to control the syncing centrally.

At the end of the day, I think it’s going to be more trouble than it’s worth.  You ultimately are trying to sync up separate systems that are not designed to be in sync, and are not aware of the other systems they are to sync to .  If it were easily done with using a wired or wireless network, there wouldn’t be any need to for multiroom audio systems to being with.