Win 7 is stretching it in a C521 to start with. As you have tried several Xonar cards they must not be compatible with the C521. I can't access the Dell downloads right now to see what motherboard chipset
drivers are needed but you can try reinstalling whatever chipset
drivers are available as that may help. Usually the chipset
drivers are not OS dependent. Also, make sure you have the SigmaTel disabled in the Setup (F2 to access the setup at boot up).
I do a lot of support on the Dell users forum and I don't recall seeing anyone else that is using the Xonar sound card with a Dell. You may also be better off with a PCI sound card rather than a PCIe sound card.