Why do you need a splitter cable? Wait, that's a headset jack, right? If so, you only get a mono mic input, so make sure the whole shebang isn't shorting one of your output channels to ground. I mean, it should work the way you're showing it (Lch out, Lch in), but you just never know.
If you have doubts about the 3.5 mm cable, they can be checked fairly easily with a multimeter.
You can right-click the level field in device properties and switch between % and dB. In my Realtek, 100% is +12 dB, 82% is +6 dB and 55% is 0 dB, and at least the positive side appears to be all digital.
Your recorded noise level appears... quite high.