Sound Problem with USB speakers - Vostro 1310

Hello,

I installes Win7 on my laptop one month ago. Everything works fine EXCEPT I now have a serious problem with my external speakers.

System:
Dell Vostro 1310 Notebook
Intel Core 2 Duo CPU T9300 2,5 GHz
3 GB RAM
Win 7 Professional 32 x
Sound(chip): Realtek ALC 268 HD
External Speakers (via USB): Teufel Concept C 200 USB

Problem:
The sound of the speakers suddenly interrupts. This can happen after 30 seconds or after 30 minutes. Seems quite random. This happens during flash videos (youtube) as well as during music (itunes, Win Media player).
If the problem occurs, the external speakers cannot even play the test beeps of the sound menu. THe test beep is not played and I have to end the test manually. The songs stop playing (really stop - the time counter does not go on). To end this situation I either have to close the browser or itunes, deaktivate and activate the speakers or just deplug the USB cable..
The USB speakers are my standard audio device. I already tried every possible configuration in the context menues (exclusive rights, amplifications,...)

Driver:
I already tried everything. The Dell driver is a vindows Vista driver. I get an error during installation. The newest driver from the Realtek homepage does not solve the problem. Even the standard Windows driver "HD Audio device" does not change the problem..

What is going wrong?

Looking forward for some help.

P.S.: Already checked the speakers with another laptop. They're fine. It must be some software-hardware problem with my laptop.