I understand that you are only receiving audio from your Slingbox on one earpiece while watching it on the iPhone. The issue might be caused by the cable connections or the settings from your Slingbox.
Let us know if the issue persists while watching the Slingbox on Watch on Slingbox.com with a computer with speakers. If that's the case, you might want to try a different set of audio cables and check that the settings on Setup on Slingbox.com.
Feel free to use this link as a troubleshooting guide: