We have a audio troubleshooting articles which would be a good place to start. I'd also delete and reinstall the SlingPlayer plugin.
Our streaming technology can sometimes impact audio/video sync, this article explains this a bit more.
Does this audio issue occur every time you use the Slingbox? Do you use any mobile apps? If so does it occur when you use them?
Let me know if this helps,
Sling Moderation Team