Have you checked the Slingplayer options? By checking Slingplayer command bar Settings | Player | Audio Settings there are four choices in the drop down; Stereo, Left Channel, Right Channel, and Mono. Is it possible that you have it set to "Left Channel"?
If that doesn't resolve it, then you might consider checking the audio output from the set-top box to the Slingbox. You can rewire the output from the set-top box or connect the output connectors into some audio equipment. The Slingbox can only hear