iPhone 14 in Firefox (still Safari under the hood of course) - no silent mode, no sleep/DND/etc., volume turned up (confirmed via other browser tab playing YT) - and still no sound from the app.
I’m guessing you may be using a JavaScript API which possibly doesn’t exist in whatever version of Safari (fuck Safari…fuck Apple’s control of browsers…etc).
My advice: Use the free tier of BrowserStack to test basic functionality on a wide range of devices (assuming they still have a free offering…). Also check “caniuse” to see if all the JS APIs you’re using are available on a wide range of devices.