Same proble here. It seems in case the FN key is pressed for three seconds, the keys stays in FN-mode. When you F10-F11 the sound is decreasing/increasing (as you described). By your comment I found out that FN can be unlocked by pressing the FN-key again for three seconds. Indead, this is annoying. I am not using it for gaming, just for typing and sometimes I want to change the sound volume.