Did you try to blacklist the driver for the onboard sound?
No...I have never have to do anything like that before. I heard of people doing it.
Where would a person go to blacklist the on-board driver?
The only time I've done it is with direction from someone else.
But if you know which module may be causing a conflict, (I am not sure that is the problem, but it is worth a try), you can put the name of the module (driver) into '/etc/modprobe.d/blacklist'. You, of course, have to be logged in as root to edit this file. You should also be able to just 'rmmod modulename' to do the same thing after booting. Then, you will probably still have to modprobe the correct driver module.