Sorry for the misleading title, but this is the simplest and quickest solution that I found.
I recently upgraded my media centre from Ubuntu 11.10 to Ubuntu 12.04, and to my great surprise, my Asus Xonar DS audio card did not let me configure my 5.1 surround setup. After some research, it seems that this otherwise fantastic audio card is not supported in the later kernels.
The solution: Downgrade to Ubuntu 11.10, and everything works fine again. 🙂