There are a small handful of things that could be contributing to your audio issue. From system and network requirements to the specific cabling you are using, there are a few things you'll want to check. I'd start with testing your network speed at the home location and the location you are viewing from, as well as testing the physical cabling that connects your set-top box to the Slingbox. Take a look at this support article, it will get you pointed in the right direction as to where you'll want to begin.
The last step suggested in this article would be to perform a hard reset and reconfigure the Slingbox unit. If it comes to that, here is some more information you might find useful.
Hope this information was helpful!