Thanks for reaching out to us here in the forums, that's a great question! Although an HDMI cable can sometimes have an impact on the Slingbox's ability to stream HD content, it wouldn't just interfere with the audio. I would suggest checking the audio connections/cables for the issue you are describing.
Here is a link to an article regarding troubleshooting audio issues:
Hope this helps!
The Sling Moderation Team
I have had my slingbox (Pro-HD) for 3+ years now with no issues. About 2 months ago I noticed that I was not getting any sound coming out of the slingbox but my picture is fine. I have tried resetting the box to factory settings with no sucess and I have also tried multiple devices to see if it was an issue with my pc, with no sound coming through. I am getting sound on my TV so it's not the cable box but it seems to be an issue with the output from the slingbox itself. Is this a hardware issue itself with the box? Or is there something else that I am missing.