can I use onboard audio and a sound card at the same time?

I just set up my new home theater and this time decided to connect my PC to my Onkyo 605. My question is this: I have a Creative X-FI sound card that I have been using to connect my PC to my PC speakers. I was planning on using the oboard Realtek sound output to connect to the receiver. I tried to connect it, but nothing happened. I switched out my X-FI connections and it worked. So either I have somehow disabled the realtek (I used it initally but switched and do not remember uninstalling driver) or there is no way to simultaneously use both sound outputs. Or is there?
Also, I am using COAX to connect my PC to the speakers. Is there a noticeable decrease in quality if I were to use the analog connections? Another solution is to have the COAX go the Onkyo and the analog go the PC speakers.
Thanks for the help!