No, it will not revert any x16 pcie lane to x8 speed. The x4 pcie lane do not share any bandwith w/ the x16/x8 lanes.
But if the gtx580 you're getting is 2x Asus gtx580 DCUII you'll not be able to install the soundcard, 'cuz it's a 3-slot.
If it's only an EVGA 2-slot 580, you're golden.