Sound travels at a speed of ~1 foot/millisecond

Oh, that's a nice approximation! Similar to Grace Hopper's famous demo of a six inch wire being about how far electrical signals travel in a nanosecond.

Wow it's insane how slow that is!

But also, I don't really have an intuition for why the speed of travel is relevant here?

It's funny that I have a natural intuition that sound is slow over long distances, yet 1 ft/s still feels astonishingly slow. And yet while I know light travels 1ft/ns, it's still astonishing that it takes 30ms to travel from London to Sydney at that speed.

sorry for late response

What I mean is that if you have multiple speakers around you, if any one is 1 foot closer or further than another, it'll be off by a millisecond already. Given that most people probably aren't going to have sub-foot positioning accuracy, sub-millisecond timing accuracy isn't critically important.

Yeah the threshold is pretty brutal, but it is enough. Experimentally, I'd say you need under 2-3ms but even at 1ms you can start to hear some phase differences.

Most of the time, I think my synchronization algorithm is actually sub-1ms, but it can be worse depending on unstable network conditions.

How are you measuring this? I'm surprised the Web Audio API scheduling system has that much insight into the hardware latency.

I was wondering that too. It’s an impressive demo when used on devices with low latency audio drivers but I’m not convinced there’s any ability to detect drift beyond this. Might be interesting to have an option to use microphones to detect and calibrate this… …but then you have the same issue of an unknown delay on the microphone input too.

If you look at professional distributed audio systems (Dante, AES67 etc) you'll find that they all require PTP support on the hardware to achieve the required timing accuracy, so yes, you need <1ms to get to the point of being considered suitable if you are doing anything which involves, say, mixing multiple streams together and avoiding phasing type effects.

However, it very much depends on what your expectations are, and how critical your listening is. If no one is looking for problems, it can be made to work well enough.