If you're not getting audio through the left earpiece on your Bluetooth headphones when using SlingPlayer, it sounds like you may be running into a cabling issue. The first thing I'd like you to do is check the red and white (audio) cables going from the OUT on the cablebox to the IN on the Slingbox.
Keep in mind that the white cable handles left audio so you may want to replace this cable just to be 100% sure that the white cable you have is fully functional.
Get back in touch if the issue persists!
Sling Media Forum Moderation Team