> Syncing networked clients to play audio at exactly the same time is a solved problem.

I was going to point out that with the variance in FM demodulation chips, using a pile of FM receivers probably wouldn't get you perfectly synced audio these days at all, even more so if it's going through usb/software/audio stacks.

Then I re-read the Ops comment and this actually seems to be a network of _transmitters_. I'm not sure what problem they're trying to solve, but I can't believe multiple PiFMs is ever the answer.

Commercial DAB radio does use single frequency networks (with tight timings and clever calculated offsets), and I am somewhat curious how analogue FM responds with regard to offset destructive interference, but this isn't that.

Please don't do this. For context, a car FM transmitter is limited to 250nW (in many jurisdictions). A Pi GPIO pin with the right bit of wire is potentially capable of 10mW or more. 40,000 times more powerful and a lot more noisy. One could be causing problems for people surprisingly far away.