I am currently trying to pre-load an MP3 file via an XMLHttpRequest in the browser and play it in the audio player. The audio file is played when set directly in the player. Therefore, I must have an error when processing the response. 

```
loadAudio : function(playerId, source) {
    var player = document.getElementById(playerId);
    var request = new XMLHttpRequest();
    request.onreadystatechange = function() {
        console.log('readyState: ' + request.readyState + ', status: ' + request.status)
        if (this.readyState == 4 && this.status == 200) {
            console.log(request.getAllResponseHeaders());
            var blob = new Blob( [request.response] );
            var url = URL.createObjectURL( blob );
            player.src = url;
            player.addEventListener('loaded', function(e) {
                URL.revokeObjectURL(player.src);
            });
            player.play();
        // Typical action to be performed when the document is ready:
        }
    };
    request.open("GET", source, true);
    request.send();
}

```

I receive the following error message:

> 
> DEMUXER\_ERROR\_COULD\_NOT\_OPEN: FFmpegDemuxer: open context failed (code 4)
> 
> 
> 

Does anyone have any idea where to find the error? 
Thank you for your answers.