Well all signals now are digital. I mean music is rarely recorded or playedback in waves (i.e. records, tapedecks, older techs). Really what matters most now is the level of reproduction quality. I mean like comptuers certain things limit quality like the quality of the recording first. Also the quality of the receiver, and perhaps one of the most important quality of the speakers. Good quality products are able to keep a steady stream of current to speakers, receivers, etc. So if you have a good stable current going from the receiver to the speakers you want a good steady current going from the computer to the receiver. The better quality sound card gives you this as well as better higher bit reproduction. Better quality, freqency, separation, pitch changes, etc. That is why you hear a lot about the quality of the DAC (digital to analog converter) on a lot of high end equipment. The newest tech is a dual ring DAC, really good but really expensive that allows for the best reproduction to date. You will notice that I kinda conflited myself by saying analog, but what this refers to is converting a digital signal (data i.e. 1's and 0's) to an analog electrical current sent in puleses to the amplifier/receiver. Hopefully this answers your question if you require further clarification, or find this hard to understand don't hesitate to ask.
Cheers.