Streamed waveform keeps changing sampling rate

Fort the stop play:

Make this line

var watch;

the very first line in your code and insert after the line in which you turn the LED1 on these lines:

watch = setWatch(function(){ // set the watch for stopping
    w.stop(); 
    setWatch(play, BTN1, { repeat: false, edge:'xxxxx' , debounce:50});
  }, BTN1, { repeat: false, edge:'xxxxx' , debounce:50});

Then add in the stop before the line with w.stop(); this line:

clearWatch(watch); // clears the watch for stopping