The PCIe on that board is x16, x1 x16 (at least in slot size). As it has a switcher card next to the pcie x1 (the little white one where you could stick a pcie x1 sound card) indicates a x16 x1 mode, where the top pci express will operate at x16 in non sli (ie with one gpu) and the bottom one will operate at x1. The middle one will always be x1 which is good for sound cards. The card can be flipped to give x8 and x8 speed on the full size slots. This probably confused you more.
Any how the pcie sound card would sit separately from the graphics cards: