Originally Posted by matthewg42
Try using alsamixer in a terminal. This is a text-mode mixer, so it's not as nice to use but it might show some extra controls which don't show in kmix. This is pure speculation on my part - I have never seen kmix omit anything, but it's worth having a look at.
Left/right cursor keys to choose which control to select, up/down to change the levels. All the controls probably don't fit on one screen, and you can continue to press the right cursor to scroll to more controls.
The space key selects channels from alternatives, e.g. using mic instead of line in (a red L R CAPTUR indicator shows at the bottom of a control when this is done). You can toggle mute with the m key. This appears as MM at the bottom of a volume control when muted, OO otherwise. The escape key exits.
Importantly, you select groups of controls with the tab key. The selected group can be seen next to "View:" at the top left. Check all groups.
Tried that as well using alsamixergui, which is a gui equivalent but no luck