I used to have an Asrock motherboard, and it wasn't until I upgraded to my Gigabyte board did I start having the issue wherein the right speaker in my 5.1 would not recieve any sound.
Figuring a failure in my speakers I set the output to stereo, tested via my earphones, left earphone would recieve both Left AND Rights output.
So this is the soundcards doing.
, older drivers, UNi unofficial drivers (Which broke my surround while using them), attempted the firmware patch, it claimed there were no devices to update (Or that they were already up to date).
In Ubuntu, Left and right will be sent as, Left>to rear left, Right> to Front left.
Disabled and tinkered with every options available, changed bit rate, disabled GX, double checked the 3.5's are in the right port.
Asus forums seem pretty dead too...Atleast I havn't recieved a reply.