"Failed to start the audio device" error on iOS about koel

[phanan]

This likely is related to mobile Safari's (including Chrome and Firefox, as they're just Safari on iOS) strict policy when it comes to the AudioContext. At this moment there's very little I can do about this, as it pops up once in a while with a new version of Safari, and it's very inconsistent (works with some and doesn't with others).

May I suggest you look at the mobile app instead? If you can't / don't want to build it, you can always buy it from the AppStore. I know it's a paid product in that case, but it doesn't have this annoying issue (as well as some others as with mobile PWA) and it's a way to further support the project.