Varying amplitude

///////////////////////////////////
// copy the Design Tool code here
///////////////////////////////////

#include <Audio.h>
#include <Wire.h>
#include <SPI.h>
#include <SD.h>
#include <SerialFlash.h>

// GUItool: begin automatically generated code
AudioSynthWaveformSine   sine2;          //xy=585,367
AudioSynthWaveformSine   sine1;          //xy=606,308
AudioMixer4              mixer1;         //xy=794,343
AudioOutputI2S           i2s1;           //xy=981,350
AudioConnection          patchCord1(sine2, 0, mixer1, 1);
AudioConnection          patchCord2(sine1, 0, mixer1, 0);
AudioConnection          patchCord3(mixer1, 0, i2s1, 0);
AudioConnection          patchCord4(mixer1, 0, i2s1, 1);
AudioControlSGTL5000     sgtl5000_1;     //xy=866,505
// GUItool: end automatically generated code

void setup() {
  Serial.begin(9600);
  AudioMemory(20);
  sgtl5000_1.enable();
  sgtl5000_1.volume(0.32);

  sine1.frequency(200);
  sine1.amplitude(0.5);
  sine2.frequency(200);
  sine2.amplitude(0.5);

}

float in, out = 0;

void loop() {

if (in > 6.283){in=0;}
else{in = in + 0.01;}
    out =  sin(in) *0.5 + 0.5 ;

    Serial.println(out);
//      sine1.amplitude(out); // why does my volume not vary when this or the next line are active?
      mixer1.gain(0, out);
      mixer1.gain(1, 0.0);
      mixer1.gain(2, 0.0);
      mixer1.gain(3, 0.0);

  delay(100);
}