Shouldnt need any additional soundcards for them to work. I think the audio comes straight from the control pod and plugs directly into your Mac.
How much did they set you back? I dont really care for Logitech's audio products...being a bit biased though, I had a few sets of Z5500s and werent that great over time. I picked up a few pairs of M-Audio Studiophile AV40's and theyre great , got them for sub $140~