Thank you for posting in the Slingbox Answers Forum! I'm sorry to hear you are having trouble controlling your A/V source with your Onscreen remote. The remote is actually not working, the mute and volume will always work. This is because it controls the volume in SlingPlayer.
Please take a look at the article below:
Hope this helps!
The Sling Moderation Team
I assume you have already done a proper Forum Search for your model number in case other people have already answered your question ?
If so and that didn't help then check out this thread: https://community.sling.com/thread/3300?tstart=0
Come back if it that doesn't help either.
(NOT a Sling Media representative, just another user)