Fix mixer gain function

Audio.h

 {
 public:
         AudioMixer4(void) : AudioStream(4, inputQueueArray) {
-		//for (int i=0; i<4; i++) gain(i, 1.0f);
 		for (int i=0; i<4; i++) multiplier[i] = 65536;
 	}
         virtual void update(void);
-/*
 	void gain(unsigned int channel, float gain) {
 		if (channel >= 4) return;
 		if (gain > 32767.0f) gain = 32767.0f;
 		else if (gain < 0.0f) gain = 0.0f;
 		multiplier[channel] = gain * 65536.0f; // TODO: proper roundoff?
 	}
-*/
 private:
 	int32_t multiplier[4];
 	audio_block_t *inputQueueArray[4];

examples/PlayFromSketch/PlayFromSketch.ino

   // turn on the output
   audioShield.enable();
   audioShield.volume(50);
+
+  // reduce the gain on mixer channels, so more than 1
+  // sound can play simultaneously without clipping
+  mix1.gain(0, 0.4);
+  mix1.gain(1, 0.4);
+  mix1.gain(2, 0.4);
+  mix1.gain(3, 0.4);
+  mix2.gain(1, 0.4);
+  mix2.gain(2, 0.4);
 }
 
 void loop() {