Issues with soundcard while streaming

[Hypernia]

Greetings,
A while ago (maybe a month or so) I had posted about having issues with my sound card. I recently discovered that my sound card was not directly the issue; or so I believe. I didn't normally mix on an external mixer when I used Traktor, but ended up having to when the software I use to stream (butt) required me to do just that. So, this is when I discovered the issue.

The issue:
When my sound card (Numark djio) is plugged in simultaneously with a cord from my main out on my mixer to my mic line in on my laptop, it generates a sound resembling feedback. I have my sound card plugged into my laptop, and have it set up with two outputs leading to my mixer's channel 2 and 3. At first, the sound is tolerable. Give it about a minute and you can already feel the wear on your ears. ALSO: I should mention that the problem only persists if both my sound card and "streaming cable" (if you will) are plugged in at the same time. I can and have been streaming records, and I imagine that cdj's would also likely solve the issue. However, I can't afford cdj's and my vinyl collection isn't quite a library shelf.

Things I've tried already:
Disabling my laptop microphone and checking to see if any boosting effects were implemented.
Updating drivers for my sound card. (I actually had to rollback the driver, as this made my sound card unrecognized on my laptop).
Checked all cords individually to ensure that they were in proper working condition.
Changed sound settings on Traktor; even setting no output signal. As long as the IO was set as my primary sound device, the problem persisted.

Any help would be much appreciated. I've had this problem for a while and I've run out of ideas on how to fix it.

[padi_04]

Have you set the mic in to behave as line in in your system settings?

[Hypernia]

Not sure; checking system settings right now. Where exactly go to change that?