Hmm I have an Asus P7P55D Pro. Not sure what the difference is having the non pro version. I know the "-e" means you have USB 3.0, but that only works when you have free PCIe lanes.
In my set up, I'm running two 5850s in 8x/8x in the top two 16xPCIe lanes - they're separated by 2 1xPCIe lanes. I have my AuzenTech Bravura 7.1 card in the second 1x lane. I then have a GT 240 in the bottom most 16xPCIe lane, which runs through the south bridge at 4x - I think you probably don't have this one.
Anyway so my advice is to make sure the sound card is in a 1x slot.