The sound card has analog inputs and outputs. Yes, some of them have MIDI as well, but, primarily, they have Microphone in, line in, and line out, all of which are analog and relatively high-impedance circuits. The 805x has only digital outputs, i.e. relatively low-impedance outputs capable of producing either the positive rail or ground, essentially, and nothing else.

You should read a few books on electronic circuits, the underlying mathematics, and, perhaps, at least one about the sound card before you proceed.

Now, it is true that you can, with considerable effort and circuitry, produce a relatively low frequency, perhaps into the upper audio range, using pulse-width modulation. You probably should, after reading the aforementioned sorts of books also read a bit about pulse-width-modulation. I suspect it will take you somewhere on the order of 5 years to accomplish all this. Is this really something you want to do?

RE