Thank you. I just wanted to set this thread to solved. After multiple times trying i refreshed the driver again and suddenly it worked. The issue was located in the Levels tab of tge Sound control panel. It was set to 6 instead of 100 (i have no Idea why) and i could Not change IT without touching the overall windows volume (there where two sliders: the common system volume and one for the headset aswell which was set to 6). After a Driver refresh both worked individually and i could change it to previous levels. Before that both sliders where synchronized in a strange Ratio which caused the low volume.
I don't know what and how i resolved the issue for sure nor do i know what caused the problem in the first place. Anyways, it works now.