amp(vol, rampTime) {
    var t = rampTime || 0;
    if (typeof vol !== 'undefined') {
      this.oscillator.amp(vol, t);
    }
    return this.oscillator.amp().value;
  }
MonoSynth amp
@method  amp
@for p5.MonoSynth
@param  {Number} vol      desired volume
@param  {Number} [rampTime] Time to reach new volume
@return {Number}          new volume value