Okay try this. Blacklist the asus_nb_wmi kernel module and reboot.
See if you have the brightness slider again and whether it works.
Try the function keys. (I have a feeling they won't work and we'll need to consider a workaround).
As for the headphones, that kernel parameter would have no effect on the headphones. I think its just a coincidence that they stopped working then. Double-check to make sure that the headphones are not muted when they are plugged in. If possible, log in as guest and try the headphones there. If they don't work, its indicative of a system issue. If they do work, then there is something wrong with your profile.
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