USB

The audio is routed over the USB.

USB-speakers are a well known and popular example of USB audio.

As it is a digital signal, it has to be converted to analogue before it could be fed into an amplifier.

USB DAC's can be found here.

Some brands offer amplifiers with USB input

Although it is changing slowly, the audio world is not very computer minded. Protocols like SPDIF are far more popular than USB. You might need a protocol converter from USB to SPDIF.

Cable length between full speed devices is limited to 5 meters. For a low speed device the limit is 3 meters.

As the signal degrades proportional to the length of the cable, a short cable is often recommended. Other say this put the PC to close to the USB-DAC.

A short cable

The USB 3 specs includes an optical connection.

Today USB can also be done wireless.

Consonance PC Wireless Digital Box

Wireless USB (50m radius)

Isolator

There are claims that sound quality improves by having a full galvanic isolation between the PC and the DAC. Some manufacturers of USB DAC's use optocouplers to eliminate the noise from the USB connection.

Meilhaus: USB Isolator EUR 289,00

Supports USB 1.1, USB 2.0 Full-Speed
Galvanic isolation by optocouplers up to 4000 V.
IP 40, class 2 compliant

Extenders

If you have to bridge a long range, there are USB extenders over CAT5 and optical.

Gefen's USB 2.0 LR USD 499.00

Extends high-speed USB devices up to 100 m over Cat5 cable.

This sets you back $499.00, a networked low power PC is cheaper.

Opticis M2-100-10 USB Extension Cable

Complete optical electronic isolation with remote power.

Stretch your USB connection with the Opticis Optical USB Extension Cable. Works on all USB 2.0 host system ports at USB 1.1 speeds.

Length from 3 to 50 m.
Extender and galvanic isolation in one.

USB Cables

USB audio is gaining momentum.
Like analogue interconnects, now you have audiophile USB cables.
Audiophile USB cables are as controversial as analogue interconnects.
It is hard to imagine that one cable deliver better bits than the other but jitter comes to cable manufacturers rescue

The Limitations of digital audio processors and cables create timing errors known as jitter, which remove portions of the audio signal and replace them with noise and distortion. Cables tend to round off the square waveforms of the signal, making them less clear to the processor, thus increasing jitter. This rounding effect varies greatly among cables and a truly superior digital audio cable can make great improvements in sound quality.
http://www.wireworldcable.com/categories/usb_cables.html

Yep, high end meets USB
Let’s go.

$0.78 - Monoprice USB 2.0 A Male to B Male 28/24AWG Cable - (Gold Plated) - 3ft

€ 11,90 LaCie 1.2 m

$85.00 - Wireworld Starlight 6 (1m)

Silver-clad oxygen-free copper

$125.00 - Kimber Kable MBUSAG (1m)

Pure Silver signal conductors

(6.1%) Silver-plated copper power conductors

(6.1%) Silver-plated copper drain and shield wire

High-Density polyethylene signal conductor dielectric

$1149 - Locus Design Nucleus

1-3ft, $120 each additional foot after three

Do you smell something fishy?