Examine source code of NotifySound

{
  "manifest_version": 2,
  "name": "NotifySound",
  "version": "1.0",
  "description": "Play a custom sound when the page's title or favicon changes (useful for feed/chat updates).",
  "icons": {
    "48": "icon-48.png",
    "128": "icon-128.png"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content_script.js"
      ],
      "run_at": "document_idle"
    }
  ],
  "permissions": [
    "storage"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{178aeafd-a83b-42d7-8aab-34602b24cf77}"
    }
  }
}