Switch the transistor how? Sound card output will switch a transistor just fine if directly connected to the base, for the positive half of the cycle it will be on, for the negative half it will be off as the base is reverse biased, a sound card (without an amp) won't be able to cause a normal transistor to go into base breakdown. You could use a diode to charge a capacitor with a base resistor to act as a low pass filter, but it would greatly depend on exactly what you were trying to switch.