script.js

let elements = {
  'a': {
    'audio': 'sounds/sound.mp3',
    'video': 'grass.mp4',
  },
  'b': {
    'audio': 'sounds/sound2.mp3',
    'video': 'sun.mp4',
  },
  'c': {
    'audio': 'sounds/sound3.mp3',
    'video': 'moon.mp4',
  },
  'C': {
    'audio': 'sounds/sound4.mp3',
    'video': 'moon.mp4',
  }
};

let input = document.querySelector('input');
let audio = document.createElement('audio');
let video = document.querySelector('video');

function keyEvent(event) {
  if (event.key in elements) {
    audio.pause();
    video.pause();

    let data = elements[event.key];
    audio.src = data.audio;
    audio.play();

    video.src = data.video;
    video.play();
  }
}

input.addEventListener('keyup', keyEvent);