Have you tried optimizing the audio in your Set Top Box? The process is a little different from box to box, but generally if you go into "Settings" or "Menu" on your physical remote, you can then access "Device Settings" and then "Audio Settings". From there, you will normally have one of two options: either a "Fixed or Variable" slider, or a button to click that just says "Optimize Audio". If you have the slider option, make sure this is set to "Fixed".
I hope this helps!
Sling Media Technical Support