USB data feed to headphone amp

Using Vista Home Premium and Lehmann Black Cube Linear + USB headphone amp.

When I first plugged in the amp everything worked fine. Mp3 files via Media Player and TV & video sound in Media Center were of audiophile quality, broader volume range, in a word perfect.

Then I connected the amp to my CD player, obviously with an analogue interconnect, and again it worked perfectly.

But when I reconnected to the PC I got no sound at all, I couldn't even hear the clicking noise made by the amp's switching mechanism. It still works when I test for sound in Sound Manager, but I get only silence from Media Player and Media Center.

Grateful for any suggestions.