Issue with external Soundcard

[Cockney]

Hello Community

Couldn't find anything yet that could solve my problem. Hope, you can help me:

I bought a thinkpad sl500 to use my VST-instruments for live playing. For this I have an external audio-interface (NI Audio DJ 2 USB) and work with Kore 2. Playing the instruments (through the external soundcard) works superb, on highest quality. The audio-interface hasn't got any issues or problems and works very fine also on other computers and notebooks.

Now the problem:
I just wanted to play an mp3-file through my external soundcard. But this is more difficult than I thought. The internal soundcard with the speakers work good. But if I choose my audio-interface in the (windows-)mixer, the Windows Media Player can't play mp3s anymore. The same problem is with the VLC-Player (there, it plays the file but there is just no sound). The external soundcard doesn't show any outcoming signal neither. The same problem is when I choose the audio-interface as the standard soundcard. When I switch to the internal speakers again, everything works properly.
Hadn't got a problem like this (until now).

Is this a well-known problem, do u have a solution? Thank you for answers.

Best regards,

Chris

(my system: sl500, 4g Ram, 2g Hz, Win Vista 32bit)