Hi DNJ
When you say the front jacks, do you mean the ones built into the case, as if so they are always stereo, likewise if the sound card is built into the motherboard the jacks on the rear are also stereo, (I don’t think you can buy a mono sound card these days) therefore providing your using a 3.5mm stereo jack plug, the only reason it would be mono is if a setting in the software of the soundcard or program was set to mono.
Best course of action is to have a rummage through the menus to see if you can find anything. (You can also email tech support of your sound card)
Hope this helps

Bill