Ah i have one of these in my car, bought it from Anyware computer accessories in Australia (they're a wholesaler).
Internally it has a USB to AC97 bridge, and a Via VT1612A Audio Codec IC in it (same as on the Via EPIA series motherboards). Audio quality is good! The only problem i have with it is that when windows or linux boots and initialises the device, the power to it drops off momentairily and it when it powers up again there's a rather loud thump that goes through the output jack. You could probably fix this by using some sort of speaker protector circuit.
They're good, i have a USB cable running from my car computer in the boot to this USB audio box which is right next to my amp.. hopefully this reduces the noise since USB's a balanced line ^^
you wrote about some USB-to-AC97 converter chip. I am a hardware developer, and I am currently in need of exactly such a part. Can you tell me the part number or at least the manufacturer of the whole device?