I don't believe you are actually sending with transferrables

[gordyr]

Please forgive me if I am incorrect but I was of the understanding that you can only send an array of ArrayBuffers as transferable objects?

In your worker.js file you have the following:

 // use transferrable objects
        var buffers = [];
        for (var i in result.outputFiles) {
            buffers.push(result.outputFiles[i]);
        }

        postMessage({
            'type' : 'done',
            'data' : result
        }, buffers);

Which causes the buffers array to be as follows:

buffers = [{
     name : filename.mp3,
     data : ArrayBuffer
}]

Therefore the object get copied rather than transferred. I'm not 100% on this as i'm relatively new to webworkers, but from all the reading I've done this appears to be the case.