waveform.startOutput on MDBT42Q seems not to start, any experience out there?

I try to play with sounds on my MDBT42Q module. It seems that w.startOutput is not doing anything. Any hint? Furthermore, event 'finish' is never emitted.

My code is below:
(sound.raw is in internal storage and has been generated following the procedure explained here)

BZ=D28;
LS=D29;
var w;
function play() {
    var wave = require("Storage").read("sound.raw");
    w = new Waveform(wave.length);
    w.buffer.set(wave);
    analogWrite(BZ, 0.5,{freq:40000} );
    w.startOutput(BZ, 4000);
    w.on("finish", function(buf) {
      BZ.reset();
      console.log("done!");
    });
}

var inited=false;
function onInit(){
  if(!inited)
  {
    inited=true;
    analogWrite(BZ, 0.0,{freq:4000} );
    pinMode(LS,'input_pullup');
    setWatch("play();", LS, {repeat:true,edge:'rising',debounce:50})­;
  }
}
onInit();