Simple method to flash an RGB LED to audio input.

beatreader.ino

/*
 * MIT License
 * Copyright (c) 2018 Sid Mani

 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:

 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.

 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */

// RGB LED on PWM pins 12, 14, 15
// Audio in on A1
void setup() {
  randomSeed(A0);
}

void loop() {
  int oldVal = analogRead(A1);
  int sensorValue = oldVal;

  int red;
  int blue;
  int green;

  if (sensorValue < 200) {
    red = random(10, 50);
    blue = random(30, 100);
    green = random(15, 120);
  } else if (sensorValue < 400) {
    red = random(10, 100);
    blue = random(30, 125);
    green = random(150, 255);
  } else if (sensorValue < 600) {
    red = random(10, 120);
    blue = random(150, 255);
    green = random(90, 225);
  } else if (sensorValue < 800) {
    red = random(100, 200);
    blue = random(90, 200);
    green = random(120, 150);
  } else if (sensorValue < 1024) {
    red = random(190, 255);
    blue = random(180, 240);
    green = random(150, 230);
  }

  analogWrite(12, 255 - red);
  analogWrite(14, 2*(255 - green));
  analogWrite(15, 2*(255 - blue));
  while ((analogRead(A1)) > (oldVal - 10) & (analogRead(A1)) < (oldVal +  10)) { delay(45); }
}