I had this problem and with a little snooping around I was able to fix it.
Open up KMix and it will show you a buch of volume controls.
This might not be the best way to go about it but it won't screw anything up, so you are safe
Start turning off the volumes one by one. If the sound goes completly off, turn it back on, if nothing happens move to the next one. Mine had a default setting of a line input on, turning it off fixed it