We need to collect some more information to better assist you with your no audio issue. What cable are you using for the cablebox OUT to Slingbox IN. Are you using component cables (red,blue, and green as well as white, and red). If you are when using component the red, blue, and green control video, and the red and white are used for audio. Component cables are a five cable system. If you are not using component please reply and let us know what cables you are using to connect the Slingbox IN to cablebox OUT so we can further assist.
Here is a basic audio troubleshooting article:
Sling Media Forum Moderation Team