Thanks for using our answers forum! I'm sorry to hear that you're having this issue where the audio and video are out of sync on Watch on Slingbox.com. It's hard to determine what exactly the issue is but if it is isolated to the one machine then it is likely the result of settings within that system, ie: audio drivers etc.
Here is an article from our support site which should assist you further with this issue you are describing:
Hope this helps!
The Sling Moderation Team