Examine source code of Silence Reminder

{
  "manifest_version": 2,
  "name": "Silence Reminder",
  "version": "1.0",
  "description": "Reminds you if a tab is playing sound but you might not be hearing it (low volume or muted).",
  "permissions": [
    "tabs",
    "activeTab"
  ],
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": false
  },
  "icons": {
    "48": "icons/icon48.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{42755c6c-741b-42c3-be42-577190fb1843}"
    }
  }
}