Unless there is a cable you're after is lurking somewhere, up for sale, on the Internet, it physically cant be done. Unless your soundcard uses a pcix1 slot.
I would have thought though, with dual dual-slot graphic cards gaining popularity and the physics card in the headlines, some company has at least thought about producing some sort of pci slot extension to allow 2 dual-gpus, a ppu and a soundcard to fit. Maybe a cable in the style of a hard-drive connection, and the ppu hanging from the roof of the case, as this wouldnt need to a pci slot in the case.
I guess this is a reason to stick with dual single slot gpu's.