Ah, I am so used to video on the web not being muted that this didn't occur to me. Thanks!

Although it then remutes if I lock and unlock, or switch away. iOS is fun.

You might need to take it out of silent mode with the physical switch on the side of the phone

Yeah, that works the first time, but then sound is gone again when you switch away to another app and back to Safari. It's really no big deal, just feels like it must be frustrating working on user experiences like this across devices!

it is web audio api thing. Apple mute the Web Audio Api when it is on vibration mode.

Confirmed. No sound if in silent mode. Works fine if you switch the sound back on.