In the really small USB device category today we find the USB 5.1 Audio Adapter that is a very tiny USB stick that adds 5.1 audio to any computer with a USB port. $10 (but they are out of stock currently).
Eliminate the need for a traditional sound card with this USB to Audio adapter. Real USB plug & play to upgrade any on-board soundcard to 5.1 Digital Surround Sound without any hardware installation. Connect a speaker or headphone directly to the adapter and enjoy 3D positional sound and virtual 5.1ch sound track. Use the USB port power directly.