esp-wemos-audioswitcher/firmware_arduino_mini/firmware_arduino_mini.ino

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

// Serial Protocol:
// 9600 Baud
// 1 char: room number 1-5
// 2 char: command v (volume) / r (relais)
// relais command:
//   3 char: a or b
// volume command:
//   3-5 char: 0-100 volume


long tick;
uint16_t relaisData = 0xffff;

void setup() {
  Serial.begin(9600);
  Serial.setTimeout(100);
  
  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);

  setVolume(1, 50);
  setVolume(2, 50);
  setVolume(3, 50);
  setVolume(4, 50);
  setVolume(5, 50);
 
}

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 & 0xff);
  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(15) | _BV(14));
      } else {
        relaisData &= ~(_BV(15) | _BV(14));
      }
      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);
}


void loop() {

  long currentMillis = millis();

  if (Serial.available() > 0) {
    String ser = Serial.readString();

    int room = ser.charAt(0) - 48;

    if(room >= 1 && room <= 5) {

      char command = ser.charAt(1);

      if(command == 'v') { //volume
        int datapin=0;
        
        switch(room){
          case 1:
            datapin=PIN_DATA_VOL_A;
            break;
          case 2:
            datapin=PIN_DATA_VOL_B;
            break;
          case 3:
            datapin=PIN_DATA_VOL_C;
            break;
          case 4:
            datapin=PIN_DATA_VOL_D;
            break;
        }
        
        int vol=ser.substring(2).toInt();
        setVolume(vol,datapin);
        
      } else if(command == 'r') { // relais
        
        char state = ser.charAt(2);
        
        if (state=='a') {
          switchInputs(room,true);
        } else if (state=='b') {
          switchInputs(room,false);
        }
        
      } // end if command check
    } // end if room check
  } // end serial available check
}