Optiplex 9020 Microphone/Line-in combo jack

Hello! 

   We have a refurbished Optiplex 9020 at our church and it's really nice!  I love how quiet it is when it's not under a load.  We bought this box to replace our old second generation i7 Audio/Video computer that died.

   The issue we're having is with the sound level coming into OBS (Open Broadcasting Software) from our soundboard. We have a Pre-Sonus soundboard connected to the back Line-in/Mic combination jack.  The sound coming in though that jack seems to be attenuated.  We have to crank every possible sound level up to max to even hear someone at a microphone, but even then it's barely audible. 

   When looking at the Sound settings in Windows it seems that the jack is being recognized as microphone vs. Line-in.  I say that because the Microphone settings are viewable and changeable and the Line-In settings are greyed out. 

So, how do I tell the computer to use it as a Line-In jack vs. a Microphone jack?  Is this a bios setting, or does it detect the type of tip that plugged in, or something simple I'm missing? 

Rob.

p.s.  (this exact setup was working with the old computer using it's Line-in jack)