One bit of feedback I don’t see mentioned. This may be an iPhone thing, but my experience is that the visuals are about a quarter beat ahead of the audio (at 60 BPM, so about 200-250ms off).
Not a big deal, but definitely enough to make it feel a bit off.
Again, could be just the nature of audio in Safari on iPhone.
http://drumpatterns.onether.com/?audio=1 http://drumpatterns.onether.com/?audio=2 http://drumpatterns.onether.com/?audio=3 <- check this one out first
You're out of luck on Safari because it seems that the important APIs aren't implemented yet: https://developer.mozilla.org/en-US/docs/Web/API/AudioContex... (scroll down). This means this cannot be made correct, e.g. there will always be a desynchronization when using high latency audio output devices such as anything wireless. Their handhelds (on which non Safari is not allowed) and their MacBooks, in wired/built-in speaker mode have excellent latency figures and we can get away with not doing anything explicit there, granted the audio is not happening somehow before the visuals, that is jarring for a human. A bit late is a lot more natural if it can't be exact.
lmk if you need further details on tight synchronization of real time audio and visuals, padenot@mozilla.com, happy to help, and congratulations on the delightful websites!