you can give the below code a try, it uses btoa

function getData(audioFile, callback) {

    var reader = new FileReader();

    reader.onload = function(event) {

        var data = event.target.result.split(',')

         , decodedImageData = btoa(data[1]);                    // the actual conversion of data from binary to base64 format

        callback(decodedImageData);        

    };

    reader.readAsDataURL(audioFile);

}