var blob = new Blob([event.target], {type: 'audio/mp3'});
Blob { size: 25, type: "audio/mp3" }
var oReq = new XMLHttpRequest()
oReq.open("GET", event.target.src, true)
oReq.responseType = "arraybuffer"
oReq.onload = function(oEvent) {
var blob = new Blob([oReq.response], {type: "audio/mp3"})
// ...
}
oReq.send()