Thanks for using our Answers Forum. I understand that your getting a buzzing sound on your audio now since you changed your cable box. I have an article that includes some basic troubleshooting steps for audio issues. Slingbox audio issues, basic troubleshooting.
Hope this helps, if not, feel free to give Slingmedia a call at 877-GoSling(877)467-5464
Sling Moderation Team