There are a few things that could impact audio and video being out of sync.
Take a look at this article, it outlines several possibilities and fixes: Audio and video are out of sync in Watch on Slingbox.com
My best guess is either you need to uninstall/reinstall the plug-in or possible just reset the Slingbox.
Recently the Slingbox website automatically update plug, causing the Slingbox 350 and 500 audio and video are not synchronized.
The problem occurs in a very computer, adjust the best high-definition viewing mode or automatic mode, change channels using the remote control, audio and video are not synchronized to produce, Slingbox site is not updated before the plug is normal.
My computer model HP Pavilon s3620tw