Examine source code of Transpose by Chordify

{
  "browser_action": {
    "default_icon": "icon-32x32.png",
    "default_popup": "popup.html"
  },
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": false
  },
  "content_scripts": [
    {
      "js": [
        "content.js"
      ],
      "matches": [
        "https://chordify.net/chords/*"
      ],
      "run_at": "document_idle"
    },
    {
      "all_frames": true,
      "js": [
        "SignalsmithStretch.js",
        "chordify-embed.js"
      ],
      "matches": [
        "https://www.youtube-nocookie.com/embed/*",
        "https://www.youtube.com/embed/*"
      ],
      "run_at": "document_end"
    },
    {
      "js": [
        "SignalsmithStretch.js",
        "chordify-embed.js"
      ],
      "matches": [
        "https://www.youtube.com/watch*"
      ],
      "run_at": "document_end"
    }
  ],
  "description": "Transposes the audio when using the transpose feature on Chordify. Also works on YouTube.",
  "permissions": [
    "activeTab",
    "https://chordify.net/",
    "https://www.youtube.com/",
    "https://www.youtube-nocookie.com/",
    "https://music.youtube.com/"
  ],
  "icons": {
    "32": "icon-32x32.png",
    "48": "icon-48x48.png",
    "128": "icon-128x128.png"
  },
  "manifest_version": 2,
  "name": "Transpose by Chordify",
  "version": "0.5",
  "web_accessible_resources": [
    "inject-songpage.js",
    "SignalsmithStretch.js",
    "inject.js"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{49892239-1fe1-4fcb-ab02-15ad4debd39d}"
    }
  }
}