Soundcards are ok ... sometimes (Score 2, Informative)

Almost all measurements with oscilloscopes cannot be performed with a sound card. However, if your application meets these ...

• Your signal is intrinsically AC coupled
• Nothing you're interested in is outside the audio band (50Hz to 5kHz or so)
• Up to a second delay for the screen update latency (depending on sound card) is ok
• Your circuit is floating, so you can tie its ground to the computer's ground
• 16 bit digitization (or worse) is enough
• No hardware triggering (all done in software)
• When dual channel, both have the same settings
• Input resistance is unimportant

You need to take a really careful look at what you're trying to do ... it is usually better to buy an oscilloscope or a PCI 'scope card. However, if you want to make measurements in the field and keep costs down, that's a lot of extra stuff to carry around.

Corrosion measurements on aircraft is an application where a sound card is appropriate, because it meets those limitations and carrying extra equipment around the aircraft is a real pain.