esp-wemos-audioswitcher/firmware_arduino_mini/firmware_arduino_mini.ino

#define PIN_DATA_VOL_A   5
#define PIN_DATA_VOL_B   6
#define PIN_DATA_VOL_C   7
#define PIN_DATA_VOL_D   8
#define PIN_DATA_CLOCK   9
#define PIN_SR_DATA      2
#define PIN_SR_LTCH      3
#define PIN_SR_CLOCK     4
#define PIN_LED          13

long tick;
uint16_t relaisData = 0xffff;

void setup() {
  pinMode(PIN_DATA_VOL_A, OUTPUT);
  pinMode(PIN_DATA_VOL_B, OUTPUT);
  pinMode(PIN_DATA_VOL_C, OUTPUT);
  pinMode(PIN_DATA_VOL_D, OUTPUT);
  pinMode(PIN_DATA_CLOCK, OUTPUT);
  pinMode(PIN_SR_DATA, OUTPUT);
  pinMode(PIN_SR_LTCH, OUTPUT);
  pinMode(PIN_SR_CLOCK, OUTPUT);
  pinMode(PIN_LED, OUTPUT);

  // shift out one time 0xffff to set all pins high. our relais boards
  // are active-low
  shiftRelais(relaisData);
}

void shiftRelais(uint16_t data) {
  digitalWrite(PIN_SR_LTCH, LOW);
  shiftOut(PIN_SR_DATA, PIN_SR_CLOCK, MSBFIRST, (data >> 8));
  shiftOut(PIN_SR_DATA, PIN_SR_CLOCK, MSBFIRST, data);
  digitalWrite(PIN_SR_LTCH, HIGH);
}

// channel is our "room" 1-5 / state for switching between A or B input
void switchInputs(uint8_t channel, bool state) {

  // 16 . . . . . . . . . . . . . . . . . . . 0

  // we use a simple switch case with some if instead of
  // tricky bitmagic to allow a more visual programming 
  // of our relais configuration
  
  switch(channel) {
    case 1:
      if(state) {
        relaisData |= _BV(7) | _BV(6);
      } else {
        relaisData &= ~(_BV(7) | _BV(6));
      }
      break;
    case 2:
      if(state) {
        relaisData |= _BV(5) | _BV(4);
      } else {
        relaisData &= ~(_BV(5) | _BV(4));
      }
      break;
    case 3:
      if(state) {
        relaisData |= _BV(3) | _BV(2);
      } else {
        relaisData &= ~(_BV(3) | _BV(2));
      }
      break;
    case 4:
      if(state) {
        relaisData |= _BV(1) | _BV(0);
      } else {
        relaisData &= ~(_BV(1) | _BV(0));
      }
      break;
    case 5:
      if(state) {
        relaisData |= _BV(9) | _BV(8);
      } else {
        relaisData &= ~(_BV(9) | _BV(8));
      }
      break;
  }

  shiftRelais(relaisData);
  
}

// control a M62429 Chip for Volume Control
// function take from here: http://forum.arduino.cc/index.php?topic=244152.0
void setVolume(uint8_t volume, uint8_t dataPin) {
  uint8_t bits;
  uint16_t data = 0; // control word is built by OR-ing in the bits

  // convert attenuation to volume / remember 0 is full volume!
  volume = (volume > 100) ? 0 : (((volume * 83) / -100) + 83); 

  data |= (0 << 0);
  data |= (0 << 1);
  data |= ((21 - (volume / 4)) << 2);
  data |= ((3 - (volume % 4)) << 7);
  data |= (0b11 << 9);

  for (bits = 0; bits < 11; bits++) {
    _delay_us(2);
    digitalWrite(dataPin, 0);
    _delay_us (2);
    digitalWrite(PIN_DATA_CLOCK, 0);
    _delay_us (2);
    digitalWrite(dataPin, (data >> bits) & 0x01);
    _delay_us (2);
    digitalWrite(PIN_DATA_CLOCK, 1);
  }
  
  _delay_us(2);
  digitalWrite(dataPin, 1); // final clock latches data in
  _delay_us (2);
  digitalWrite(PIN_DATA_CLOCK, 0);
}


uint8_t volume = 0;
uint8_t room = 0;
bool flag = false;

void loop() {

  long currentMillis = millis();


  if(currentMillis - tick > 500) {

    setVolume(volume, PIN_DATA_VOL_A);
    setVolume(volume, PIN_DATA_VOL_B);
    setVolume(volume, PIN_DATA_VOL_C);
    setVolume(volume, PIN_DATA_VOL_D);

    volume++;
    if(volume > 100) volume = 0;

    switchInputs(room++, flag);

    if(room == 5) {
      room = 0;
      flag = !flag;
    }
    
    tick = currentMillis;
  }

  

}