Comment 89 for bug 1886341

Actually, the use of built-in speaker and headphones at the same time happens only if I boot with headphones plugged in; and the way to mute the speakers but not the headphones is simply to unplug and reinsert the headphones. So that's fine. (It's different from what I'm used to, but acceptable behavior IMO).
And 5.6.0 is no different from the new testing kernel in that regard.