Jan Henrik wrote this Instructable detailing the build of his DIY USB oscilloscope with signal generator:
In this project I want to show you how to make an easy and cheap USB Oscilloscope.
Specifications:
Price: 20$
Frequency: 2-20000HZ
Resolution: 16 Bit
Voltage range: +-7V / with probe +-70V
Channels: 2
X-Y Mode
Signal generator: Sine, Square, Triangular and Sawtooth 0-20000Hz
…a.k.a. “how to use your sound card to input/output sound”.
Most modern sound cards support 96ksps, and many even 192ksps. Soundcard Oscilloscope seems limited to 44ksps. GoldWave and Audacity support 192ksps.
http://nerdralph.blogspot.ca/2014/05/pc-storage-oscilloscope-and-logic.html
If you are dedicating a USB audio codec to oscilloscope use, then I’d suggest removing the dc filter input capacitors for better sampling of unbalanced input signals.