Having run a few experiments, I am pretty sure that for the Librem 14 the state of the headphone jack (plugged in or not plugged in) is not passed to the operating system. If so, this prevents the operating system from switching between the speakers and headphones when you plug in the latter.
If I am correct regarding this observation, it would be good to know if this can be solved with a firmware upgrade in the future. As while it is perfectly fine to switch the speakers on and off manually, it can potentially lead to rather embarrassing situations if you fail to notice that your speakers are still on when you plug in your headphones and blast your music or video call across the room.