It is possible you just need to Optimize or make a simple adjustment to the Audio settings within your set top box so that both the TV and Slingbox are easily able to provide the audio feed.
From your home location, and using your physical set top box remote, access the "Settings" through either the "Menu" or "Settings" button (the remote itself will determine which button options it has). You should see "Device Settings" or something to that affect from within this menu; enter this and then proceed to Audio Settings. You may see an "Optimize" option towards the bottom of this menu, if so, choose this and then test the Slingbox to see if the Audio has returned.
If you do not see an "Optimize" option, check to see if you have the option to pick between Variable and Fixed audio, and ensure this is set to Fixed. Once this is on Fixed, test the unit again.
I hope this helps!
Sling Media Technical Support