An extremely low - virtually unnoticeable delay I believe does exist often. If things are working correctly you do not notice. But sometimes it isn't working right. TV's generally have a delay or sync setting for that to match it better. And sometimes changing channels back and forth fixes it. Using the PCM setting can sometimes fix it but that defeats digital sound settings.
I don't use Sling, I have not seen posts about sound sync issues while streaming. If you are streaming to a TV and have the issue I would check the TV sound settings.