My 5870 had a strange conflict with my Auzen Forte sound card. Audio playback was choppy, faded in/out, randomly switched between 2.0/5.1. As soon as I tried my mobo's onboard audio all those problems were gone. I now use an Asus D2X and it works great. As for the 5870, it's easily the best card I've ever owned. I was in the Nvidia camp since 2004, but now play for the red team.
You do know the card (5870) provides HD audio via HDMI and has audio drivers included in the full version CAT driver package, I had problems with this on my X-FI card, kept switching my speaker configuration back and forth til I went in and manuall unistalled the AMD/ATI hi-def audio drivers.