Luikki - yes that was the problem, the headphone channel in KMIX does nothing.
However - problem solved (internet trawling), a known Acer laptop issue with Alsa sound which is solved as follows:
> Browse and Configure Hardware
> Select audio controller (either one)
> Set current driver options
> model - acer
KMIX will now have "Surround" and "Front" channels where surround is the laptop speakers and front is the headphones. Funnily enough the headphone channel no longer has a slider and so needs to be removed.
*** I have read about a number of sound issues. If you have an Acer maybe the above fix and not using the "obvious" channels is a solution.
all the best,