Yeah, it's actually surprisingly tricky to get sound to persist with the screen off, especially on iOS, but I managed it in the end.

I'm testing a PWA version at the moment too so it'll be installable to your home screen - the test version at https://test.ambiph.one is PWA-enabled if anyone would like to try it out

Yeah, this is even better. Maybe add 'reset' button that will reset all sounds to "not selected".