Sound Visualization v.1.0
The first in my series of audio visualization tools written using p5.js, a Javascript variant. If the embedded window above is not working, please click through to the interactive sketch at: https://www.openprocessing.org/sketch/755040
This code was written with FreddieRa's Audio Tips & Tricks Instructions. I riffed on them for the further visualizations in this series. 
function setup() {
    createCanvas(windowWidth, windowHeight);
    background(255);

    
    //sound input
    mic = new p5.AudioIn()
    mic.start();

    
    //measures amplitude via mic
    amplitude = new p5.Amplitude(0.5);
    amplitude.setInput(mic);

    
    //auto-levels volume
    amplitude.toggleNormalize(1);
    
    //generates a spectrum for sound to appear on
    fft = new p5.FFT();
    //tells spectrum to use mic
    mic.connect(fft);
 
   
    //higher frequencies than this are sort of useless
    number = 200;
}

function draw() {
    //basics
    background(255);
    noFill()

    
    //records the spectrum for visualization
    spectrum = fft.analyze()
    
    //for loop! this shows sound as a series of rectangles
    for (let i = 0; i < number; i++) {
        x = map(i, 0, number, 0.25, 0.75)*width
        y = height/2
        w = (width*0.5)/number
        h = -spectrum[i]
        rect(x, y, w, h)
    }
}
Sound Visualization v.2.0
Attempting to make a more visually-pleasing audio-to-graphics experience, I coded the spectrum to appear in ellipse-form. https://www.openprocessing.org/sketch/755396

function setup() {
    createCanvas(windowWidth, windowHeight);
    background(255);
   
    mic = new p5.AudioIn()
  mic.start();
   
    amplitude = new p5.Amplitude(0.5);
    amplitude.setInput(mic);
    
   
    amplitude.toggleNormalize(1);
   
    fft = new p5.FFT();
    //tells spectrum to use mic
    mic.connect(fft);
   
    number = 200;
}
function draw() {
    //basics
    background(100);
    noStroke();
   
    spectrum = fft.analyze()
    
    //for loop! this shows sound as a series of very skinny ellipses
    for (let i = 0; i < number; i++) {
        x = map(i, 0, number, 0.25, 0.75)*width
        y = height/2
        w = (width*0.5)/number
        h = -spectrum[i]
        ellipse(x, y, w, h)
    }
}