Common audio cards use so-called single bit D/A converters; a single positive reference voltage is turned on and off in an organized manner at speeds well into the megahertz. Because the output is on only half of the time, the actual voltage output averages to a value that is about half of the reference voltage when the card is idle. As the digital signal goes positive, the ref voltage is turned on for a longer time, and less time for a negative signal. The bottom line here, is that an audio card produces a DC idling voltage with no signal applied, and the average DC value moves up and down from that median value (but is always positive) as the digital signal is applied- all you need for audio purposes is a DC blocking capacitor in the signal path so that you can get AC into your speakers. For a laser app, you need DC response with the ability to swing the voltage equally positive and negative, so the capacitors have to be jumpered out. The correction amp is just a 2-input summing amplfier; an adjustable DC voltage is applied at equal but opposite polarity and is summed together to cancel out the soundcard's idling voltage, and provides the ability to adjust overall gain independently.
