You can use a soundcard to send control voltages to analog requipment, however most soundcards incorporate DC-BLOCKING CAPACITORS. These are a type of high-pass filter that blocks steady signals like Pitch and gate information.
There are 3 options:
* Purchase a soundcard with no DC-Blocking. http://www.expert-sleepers.co.uk/siwacompatibility.html
* Bypass the DC blocking capacitors.
* Use AC Encoding. This is a clever idea where you AM modulate your gate/CV signals so get them past the blocking capacitors, then you construct a box/cable to rectify the signal to remove the high frequency 'carrier'. http://www.expert-sleepers.co.uk/siwaacencoder.html