Xonar U7 works perfect at home, where I use my desktop headphone as a source of sound and set it at line-in. I use a recently purchased Lenovo notebook with Intel i5 to record a powerpoint presentation with voice-over and for a software, I use Movavi Screen capture Studio. The problem arises when I bring that recording equipment to my church and plug a cable from the church mixer's line output into Xonar's mic-in. I believe the only available output with no mark has line-level, as I have recorded, with no trouble at all, my preaching with Zoom H2, connecting to its marked line-in jack. Xonar however doesn't give me line-in option, so I have no choice but to choose mic-in. I disable Lenovo's system mic, but cannot stop all the noises outside coming into my recordings, and the recording of my preaching itself is almost unrecognizable, as if the church mixer's output is mic-level rather than line-level. Overall the quality of sound is terrible. I tried tweaking around the equipment but after all could make it work.
First of all , check your cables at the church, secondly check your drivers / software etc.
Maybe consider buyng a cheap card with some real jack mic/line input.
I think you should be ok recording the line in signal if everything else i mentioned is ok (cabling & software) but in no means this is a hi end format.
Also be sure to use the right output from your card , use the buttons to give some gain to the sound.
Your question is very confusing and seems to contradict itself, but I'll try to answer with a quick, general reply.
It sounds like your equipment is being overloaded by the line level signal comming from the church mixer. Are you sure the Xonar's input is set to line-level?
Maybe the church mixer is a powered mixer?