[alsa-devel] explicit commandline control of speakers vs headphone output

It appears that when I have a headphone plugged in, the speakers are automatically disabled and the output sound is sent to the headphone.

However, I want to explicitly control (e.g. via a script or commandline - not a GUI) whether the sound from an (arbitrary) application is going to the speakers or to the headphone even when the headphone is plugged in. (When I say "arbitrary application" that means I can't alter the code of the application itself).

Is this possible?

(I have heard that ALSA mutes the speakers when it detects plugged-in headphones. Is switching between them as simple as using e.g. amixer(1) to unmute the speakers and mute the headphones and vice versa, or is there more to it.)