We have an iPad Air 2 running iOS 9.3.2, and I tried your demo in the Safari browser as it's the most convenient way to test. I refreshed 10 times and it worked fine every time. I also tried switching tabs and going to the home screen and returning, it always worked fine.
What version of iOS is your iPad running? This sounds almost exactly like a past sample-rate/audio distortion bug we had with iOS. The bug was actually in iOS and not C2, so we could not directly fix it, but we found a workaround (which should be in place in r227) and reported the underlying issue to Apple. Apple fixed the underlying issue but I don't know if it's actually made it to the release versions of iOS, or if it has, which version it went in to.
I'd add it's odd you use "is touching" followed by "trigger once" - that's the same as "on touched", and we have special provisions for the first "on touch" event that do things like unblock audio to work around iOS's restrictions. This is applied regardless of which events you have in C2, but I thought I'd mention it in case it does somehow affect it.