Hello Sylvia, thank you for your input.
the device are USB speakers (Nubert nuPro A-10) and the internal sound device is the UAC3556B (USB ID: VID_0754D, PID_3556 REV_008).
My system is an Intel Core i7 4771, 16GB Ram, Windows 7 x64 (up-to-date), Curcial M500 960GB SSD, Intel IGP and now NVIDIA 770 4GB VRAM, NVIDIA with the Intel DH87RL Board (Bios 322).
Thanks!