When I looked into "USB Soundcards" I found they are based on chips that combine an op-amp mic preamp, A to D converter, and USB interface with power drawn from USB. The converters were 16 bits at 41 or 48 KHz. Fidelity was sacrificed for low cost and minimum external parts count. Performance ranged from acceptable for video chat, to really awful. I didn't experiment further, since even the cheapest XLR input interfaces offer better audio.
