You're right; current behavior verified here, but I think I was remembering the behavior correctly, indicating Nokia changed where that policy is implemented.

I think it may be related to multimediad, which showed up at or around Diablo and enforces Nokia sound policy all over top of some (previously possible) BT/alsa tweaks. (In Chinook, for example, I could listen on BT, external jack, and internal speakers siumultaneously.) Unfortunately, that seems to be closed source, so some effort would be involved in figuring this out. With both sets of earbuds I use, or wired speakers, I never need the volume above ~50%, so I didn't notice. (I do crank the internal speakers to 100% and jack up the DMA 4 notches when cycling with no phones, but that's unaffected.)

What would be nice is a flexible, configurable variant of multimedia, which lets the local administrator (i.e. me) control policy, so I could have multiple profiles defined; one could go full volume with speakers, 40% with phones, another would leave phones alone, but mute (instead of switching to speakers) if they come unplugged, and so on. But it seems the policy is implemented in code, not config, and the code is closed. I guess forcing your weird sound policy down users' throats is what's known as a value-adding differentiator...