A sound card is nothing more than a DSO and a signal generator rated for streaming input/output with a bandwidth of 22khz. Modern sound cards have at least 2 stereo inputs (CD and line in) sometimes a stereo mic in, and many have more than one extra line in for additional CD or AUX-IN. The crosstalk is pretty bad, but can be adjusted for.
Although you can sample up to 96khz on most modern sound cards there are filter stages that limit the actual bandwidth to 0-20khz or so the falloff is rapid, the extra sampling rate over the available bandwidth for a sound card is to smooth out harmonic distortion (particularly in reproduction not sampling) Although for really pure audio samples a higher sampling rate for recording can pick up harmonics that might otherwise be missed.
The main disadvantage of a sound card is it is strictly AC coupled, although you may be able to make a chopper circuit to provide a DC bias, but that requires special hardware on the sending/receiving end and special hardware on the receiving/sending end. Also the voltage range is limited to LINE-IN input ranges (about 2 volts max) So you have to protect it with external hardware, or just never over drive it.