Comment 13 for bug 253422

After adding model=6stack-dig and restarting, you get a new tab in the volume control, called "Switches". There is just 1 tickbox with the text "Headphone", enabled by default.

If you enable the tickbox you get sound from both the headphones and the laptop's speakers (same behaviour as without model=6stack-dig). If you disable it, you always get sound from the headphones, even if they're not plugged in. If they are plugged in you get the expected behaviour (sound only through the headphones) but if they're not you don't get any sound at all.