It's usually a pretty simple fix for audio issues such as your's. Here's a support article with some helpful steps. And, as the article suggests, if you run out of options, I'd recommend factory resetting and reconfiguring the unit. I've also included some articles on how to do that, in case you need it.
Don't let the name of that third article fool you. That will walk you through the steps of reconfiguring your Slingbox 500, should the troubleshooting wind up there.
Hope this info was helpful!