Rather than use kmixer, use the CLI bits for alsa. Open up a term via ssh or locally with 'CTRL' 'ALT' "F2" (get back by doing 'CTRL' 'ALT' 'F7')
Run:
alsamixer
Will give you the sliders to set your outputs. Once happy, hit 'ESC' key to exit application. Then to save it do a:
Run:
alsactl store
This will make your settings stick on a reboot. Keep in mind, that if you use the "Rebuild Image" option from the web admin, your settings will have changed again.
Regards,
Seth