Sling Media (not Slingbox, that is the name of the device) don't provide e-mail support, you have to phone them and pay $30. That is why you are better off just using this forum. There are users on here who been around Slingboxes a lot longer than the support staff.
In your case your first step is to remove the Red/White cables from the back of the Slingbox and plug them into the TV. If you get sound then you know there is a Slingbox problem, if you don't get sound the problem is with your AV Device.