I have a 5.1 capable sound card (Intel ICH5 driver, Realtek AC'97). I am using the Monsters Inc. THX 5.1 test, and everything works fine except for the right rear surround speaker. Every other speaker plays fine except that one. I did some switching of cables, and it works when it's plugged into the rear left plug on my subwoofer, so I don't think it's a hardware issue.
XINE engine parameter for device.alsa_passthrough_device
I'm using Mepis (Debian testing/unstable) with Kernel 2.6.10