Not an answer, but we have found from experience that if the HDMI input is linked to the Component input (as recommended by Sling), then the Audio aspect of the HDMI cable is ignored and the RCA Red/White Audio input is used. Might be worth re-running the Configure Inputs routine to make sure they haven't accidentally become connected, i.e. reset the Component input.
I'm sorry to hear you are not hearing any audio with your Slingbox. If you check out the link below, it provides step by step troubleshooting to help resolve the issue. It is best if you can have someone perform these troubleshooting steps from the home location:
Hope this helps!
The Sling Media Moderation Team