There is some amount of the "Focus follows brain" problem here. What we want is for things to do what we meant, all the time, and in this case it's very possible that the visitor wanted to hear the music. It is not practical (without yet to invented technology) for that to work so we have a substitute - there's a switch and you should remember to press it.

"Focus follows brain" is how everybody wants windowed UIs to work. When I type on the keyboard the letters go where my brain thought they should go - duh, but of course that's unimplementable, so the Windows UI provides "Click to focus" - if I click on a Window the typing goes there until I click another window, meanwhile some Unix systems do "Focus follows Mouse" - if I move the mouse over a Window then my typing goes there even without clicking. Neither is what we actually wanted, both are trying to approximate.

media sound is generally unaffected by the silent mode toggle, which apple suggests is only for notifications. but the toggle inconsistently affects media, muting some things but not others. it's incredibly frustrating. android has much better audio controls for notifications, media, alarms, and vibrate.

Because silent mode is for the notifications. App volume has its own dedicated buttons.

The phone will still make sound if I launch a music app, why is a web page different?

And I hate web pages making sound! But the UX is confusing, and it’s changed over the years, seemingly without reason.

Iphones now have a software toggle as well, which may have coincided with the shift from “mute ringer” to “mute (almost) everything” that came with the multifunction button.