The Crosshair manual shows the audio slot as "Audio" not as a PCI slot.
The PCI slot marked as soundcard slot is necessary 'from asus' in order for the riser card to work. Asus speculates that people who buy an ROG board will use the given riser card solution, thas why they've marked the first PCI-E x1 slot as sound (in the manual). For the ROG boards, you can disable the onboard sound via BIOS and then you populate the PCI-E x1 (mean't for the riser card) with any x1 slotted soundcard of your choice.
Before we get on to answer, the onbard vs. dedicated soundcard...we'll need to know your speaker setup.
getting a good soundcard and hooking it up to a cheap speaker setup won;t show much of a gain in terms of sound quality. And your mem config is a lil dry to go on info :P
Oh and if your interested in switching the riser to another board, think again, people on other forums did it with no success. The risercard needs a designated onbaord chip and a slot that recognizes the riser cards signature. Using the riser card on another mobo will just waste a slot in your PC case.