setInterval in module not working

This is about object reference.

change this. to SOUND. in line 21.

setInterval(function(){SOUND.beep(freq,time); }, 1000);

or add ‘that’

function SOUND(pin,volume){
    this.PIN = pin;
    this.VOLUME = volume;
    var that = this;
}

SOUND.prototype.multibeep_works_not = function(freq,time,nr){
  setInterval(function(){ that.beep(freq,time); }, 1000);
};