Sound-controlled Neopixel LED Lights

#include <FastLED.h>
#define NUM_LEDS 60
#define DATA_PIN 6
#define AVG_NOISE_LVL 970
#define MAX_NOISE_LVL 1023

CRGB leds[NUM_LEDS];

int numLedsToLight = 0;

void setup() { 
  FastLED.addLeds<NEOPIXEL, DATA_PIN>(leds, NUM_LEDS);
  FastLED.clear();
}

void loop() {
  int val = analogRead(A0);
  numLedsToLight = map(val, MAX_NOISE_LVL-AVG_NOISE_LVL, 0, 0, NUM_LEDS);
  int brightness = map(val, MAX_NOISE_LVL-AVG_NOISE_LVL, 0, 0, 255);

  for(int led = numLedsToLight; led >= 0; led--) { 
    rainbow_wave(10, 10);
  }
  delay(10);
  FastLED.setBrightness(brightness);
  FastLED.show();
  // First, clear the existing led values
  FastLED.clear(false);
}

void rainbow_wave(uint8_t thisSpeed, uint8_t deltaHue) {     // The fill_rainbow call doesn't support brightness levels.
 
// uint8_t thisHue = beatsin8(thisSpeed,0,255);                // A simple rainbow wave.
  uint8_t thisHue = beat8(thisSpeed,255);                     // A simple rainbow march.
    
  fill_rainbow(leds, numLedsToLight, thisHue, deltaHue);            // Use FastLED's fill_rainbow routine.
 
}