Thank you for posting your question to the Sling Media Community. Based on the issue you are reporting, it sounds to me like you are experiencing HDCP. HDCP stands for High-Bandwidth Digital Copyright Protection, and is a form of content protection utlized by many Cable/TV providers nationwide. Generally, it is caused by an HDMI cable being involved in your home entertainment ecosystem. What it boils down to is this... If you have your Slingbox configured to a specific output from your cable box (that isn't HDMI), but have an HDMI cable running from your set-top box directly to your TV, your television signal may be disrupted. This would explain why you can hear sound, but do not have any video when attempting to watch. If this sounds like how you have your Slingbox arranged in your home, try removing the HDMI cable from the situation. If you want video signal to your TV without using the HDMI, I would suggest using the pass-through option by connecting either component or composite cables from the outputs on the Slingbox to your television set. When setting up a pass-through, just ensure you are using the same type of cables going from your cable box to your Slingbox as you are using from your Slingbox to the TV.
There is, however, another possibility. There is a slim chance that your Slingbox could be configured to the wrong video input. If you have configured your device to use the component inputs, but accidentally have the composite inputs connected, this will cause a disruption in your video signal. Both component and composite inputs use the same Red and White audio inputs for sound. If removing the HDMI cable from your home setup does not resolve the issue, reconfigure the Slingbox, and ensure that the correct inputs are selected in the setup that you physically have connected between your unit and the set-top box.
Should the need arise, and should you experience any issues with reconfiguring the device, here is an article that I feel will be useful.
Granted, the title of the article might seem like it doesn't apply, but all of the setup and reconfiguration steps are listed here.
Hope this information was helpful!