I was wondering if it was possible to unmute the user's computer in Javascript if the user had set the volume to muted.
No, this is not possible - if any webpage could change the settings a user's operating system had for sound, that'd be quite user-unfriendly. In a browser, Javascript's potential is mostly restricted to just the browser, unless the user explicitly gives permissions otherwise (like for notifications, microphone, camera, etc). The ability to mute/unmute/change system volume is not one of the APIs that exists, at least not yet.