Would it work properly? Would it sound weird? Should I save the 20€ and go only with the headphones which are stereo?
And if it is important the headphones are ,,ROCCAT Syva" and the soundcard is ,,ROCCAT Juke".
I got the adapter ,, from single 3 pole 3.5mm to x2 2 pole 3.5mm " so it goes into the soundcard.
That soundcard is virtual 7.1, not true 7.1. I don't even think single 3.5mm jacks can do anything above stereo. 7.1 and 5.1 are only a thing because of speakers. Stereo gives you the same positional audio. That soundcard will just add separation to the sound to simulate 7.1.
You wouldn't be able to use the inbuilt mic in that headset because the soundcard has a separate headphone output and mic input connector.
Don't bother with that soundcard and get a decent pair of in ears; Preferably something not manufactured by a gaming peripherals company.