PC Magazine has a review on the Hercules Muse Pocket USB external sound module. The unit offers paltry sound output, with noise troubles at -32dB and a problem switching from 5.1 mode to stereo ...

It's newest product, the Xonar U7 MKII, is a compact USB sound card with a built-in headphone amplifier. The Xonar U7 MKII is mostly intended for laptops, though it can be used with desktops as well.