Possibly, although first you need to get your terminology correct. The Yellow wire is Composite (SD) and the Red/Blue/Green wires are component (HD).
The most likely reason for the lack of video is HDCP (Google it). To test it, I assume you are using an HDMI cable from the cable/satellite box to the TV ? If so, throw it away and feed the TV from the component (red/green/blue) OUT on the Slingbox.
There are three possibilities as to why you are not getting any video, but do get sound. The first and simplest is that you could be setting up your Slingbox to the wrong source. If you are choosing composite (yellow) during the setup, you will not get any video if you are using components (red, blue, and green).
The second possibility could be due to the resolution on your cable box. Only certain resolutions are compatible and can be fixed by switching to a different resolution.
Lastly, you could be experiencing a form of copyright protection if the above did not resolve your issue. The links below provide more information. Hope this helps!
- Slingbox.com - Video resolutions supported by Slingbox M1/M2, Slingbox 350, & Slingbox 500
- Slingbox.com - Troubleshooting "No Video Signal" errors during setup
Sling Media Moderation Team