iPhone X iOS 15: Sound not playing


we faced some issue with sound on an iPhone X with iOS 15.1. It isn’t playing at all.

We have an audioListener on the camera. The sound is not positional and not on autoplay. It should start with an animation after clicking a button. It’s an mp3 file with length 74 sec.
On Android the sound is working just fine. Even on an iPad the sound is playing.

I know there a already several topics in the forum regarding this issue, but I tried everything suggested there.
I also start and stop the audio after first user interaction like this:

AudioStart.prototype.initialize = function() {
    var app = this.app;
    var entity = this.entity;
    var resumeContext = function () {
        window.removeEventListener('mousedown', resumeContext);
        window.removeEventListener('touchend', resumeContext);
    window.addEventListener('mousedown', resumeContext);
    window.addEventListener('touchend', resumeContext);    

We also tested the tutorials for audio and there is also no sound playing on the iPhone X.

Is this a device specific problem or is there a possible workaround?

Thanks in advance.

Dumb question from me: Is the physical mute switch on the phone been toggled?

Edit: Checked this project against an iPhone 8 with iOS 15.0 and can hear the audio: https://launch.playcanvas.com/477031?debug=true

Sound is playing on my iPhone X with iOS 14.2 and on my iPhone 12 Pro with iOS 15.1.