microphone to LED value code:

#include <inttypes.h>
#include <avr/io.h>
#include <util/delay.h>

#define LED_BIT PD4

/*
 * get_adc
 * Return the 10bit value of the selected adc channel.
 */
uint16_t get_adc() {

 uint16_t value;

 // warm up the ADC, discard the first conversion
 ADCSRA |= (1 << ADSC);
 while (ADCSRA & (1 << ADSC)); 
 value = ADCW;
 
 ADCSRA |= (1 << ADSC);    // start single conversion
 while (ADCSRA & (1 << ADSC));   // wait until conversion is done

 return ADCW;
}

int main(void) {

 uint8_t i = 0;
 
 DDRD |= 0x1c; // PD2-PD3: col 6-7, PD4: debug LED

 // select channel
 ADMUX = 5;

 // ADC setup
 ADCSRA = 
  (1 << ADEN) |      // enable ADC
  (1 << ADPS1) | (1 << ADPS0);  // set prescaler to 8 
   
 // say hello 
 for (i = 0; i < 5; i++) {
  PORTD |= (1 << LED_BIT);
  _delay_ms(10);
  _delay_ms(10);
  _delay_ms(10);
  _delay_ms(10);
  _delay_ms(10);
  PORTD &= ~(1 << LED_BIT);
  _delay_ms(10);
  _delay_ms(10);
  _delay_ms(10);
  _delay_ms(10);
  _delay_ms(10);
 }
 _delay_ms(10);
 _delay_ms(10);


 while (1) {
  
  if (get_adc() > 180) {
   PORTD |= (1 << LED_BIT);
   _delay_ms(10);
   PORTD &= ~(1 << LED_BIT);
  }   
  
 }

 return 0;

}

Shift Register code:

//Pin connected to latch pin (ST_CP) of 74HC595
const int latchPin = 8;
//Pin connected to clock pin (SH_CP) of 74HC595
const int clockPin = 12;
////Pin connected to Data in (DS) of 74HC595
const int dataPin = 11;

char inputString[2];

void setup() {
  //set pins to output because they are addressed in the main loop
  pinMode(latchPin, OUTPUT);
  pinMode(dataPin, OUTPUT);  
  pinMode(clockPin, OUTPUT);
  Serial.begin(9600);
  Serial.println("reset");
}

void loop() {
  // iterate over the 16 outputs of the two shift registers
  for (int thisLed = 0; thisLed < 16; thisLed++) {
    // write data to the shift registers:
    registerWrite(thisLed, HIGH);
    // if this is not the first LED, turn off the previous LED:
    if (thisLed > 0) {
      registerWrite(thisLed - 1, LOW);
    } 
    // if this is  the first LED, turn off the highest LED:
    else {
      registerWrite(15, LOW);
    } 
    // pause between LEDs:
    delay(250);
  }

}

// This method sends bits to the shift registers:

void registerWrite(int whichPin, int whichState) {
  // the bits you want to send. Use an unsigned int,
  // so you can use all 16 bits:
  unsigned int bitsToSend = 0;    

  // turn off the output so the pins don't light up
  // while you're shifting bits:
  digitalWrite(latchPin, LOW);

  // turn on the next highest bit in bitsToSend:
  bitWrite(bitsToSend, whichPin, whichState);

  // break the bits into two bytes, one for 
  // the first register and one for the second:
  byte registerOne = highByte(bitsToSend);
  byte registerTwo = lowByte(bitsToSend);

  // shift the bytes out:
  shiftOut(dataPin, clockPin, MSBFIRST, registerTwo);
  shiftOut(dataPin, clockPin, MSBFIRST, registerOne);

  // turn on the output so the LEDs can light up:
  digitalWrite(latchPin, HIGH);
}
Yu Wei Chiang