As Michaal says, the bandwidth of a Soundcard is going to be of very little use in looking at digital signals!

Clearly, the bandwidth of a Soundcard is going to be limited to the audio range - which commonly means something like 20Hz - 20kHz.

Digital signals have a lot of harmonics to give those sharp edges - and a Soundcard is going to have trouble even seeing the fundamental of most signals involved with microcontrollers...!

There are plenty of (relatively) cheap PC-based oscilloscopes on the market these days - and even some quite good standalone instruments; eg, take a look at http://www.rapidonline.com/Tools-Fas...09/kw/owon

Of course, with test instruments, you still get what you pay for...