Cheap C-Media (?) sound card
This USB sound card that presents itself as C-Media is one of the cheapest sound cards available, usually in the price range $1-2. I've tested it for a short time.
VID=0x0D8C PID=0x000C Product string: C-Media USB Headphone Set Audio Device Class + HID (composite device)
Crystal frequency: 12 MHz. 100uF/10V capacitors are separing headphone output. Audio input: mono. Two "live" contacts of mic jack are shorten together. Noise with nothing attached to input: ~30 LSB (surprising low). AGC setting (only one available in adv settings) works like a mic boost rather while gain control for recording seems to have no effect.
In a short time I wasn't able to find any point on PCB that would allow DC voltage sampling - it's possible that this card is not a good candidate for PC oscilloscope.