Actually, the A/D is already done in the audio card, all the software does is scale the results and display them. PC audio cards go up to 190khz sampling rates at 8/16 and some 24 bits. You can use simple attenuation circuitry to scale the input voltage range you're looking for but PC sound cards are usually capacitivly couples so you're out of luck if you want to monitor a signal that has a significant DC component. PC sound card scopes will do in a pinch, but if you really want to play with a scope there's no excuse but to go out and buy one. Even a 10 year old model off e-bay for 50 bucks or so. A lot of sound cards have built in gain stages though (for microphones) that usually provide 20db or so of gain which gives you some really good sensativity for low level signals. The upper voltage limit on the input of a sound card is around 1 or 2 volts tops, it can be attentuated internally to some degree but I don't know how that effects the A/D conversion accuracy and noise floor.