Sorry to hear you are having audio issues. Did anything change recently in your setup? New router? New browser version etc?
This article: Slingbox audio issues: Basic troubleshooting walks through some of the steps to try.
Some set top boxes also have an audio adjustment option, where audio can be optimized. Check to see if yours has that setting option. I'd also do a reset on your Slingbox and see if the issue persists.