The SlingStudio hub's audio line-in port requires a stereo input. If you have a mini plug that outputs mono audio, then it's likely that this mono audio will only play over one channel when you use it with your SlingStudio hub (likely the SlingStudio hub's left channel.)
For audio from a mono plug to work properly with your SlingStudio hub, you'll need to also use some kind of converter plug or cable that can convert the mono audio to stereo (so that the mono signal plays equally over both left and right stereo channels).
Regardless of whether the source signal is mono or stereo, the SlingStudio hub audio input requires a signal over both stereo channels. For more information, see the article below.