Takže je to jasné. Dodatek v manuálu.

(Note 2) When the second PCI Express x16 slot (PCIE_16_2) is in use, the three PCI Express x1 slots become unavailable.

Takže buď sehnat PCI zvukovku, nebo se pokusit tu RAID kartu narvat do PCIE 1x, ale to nevím jak by fungovalo.