I also hear that ALSA has some support for multiple applications per device nowadays, though I understand it is much less pleasant to use than a fully featured sound server.

FYI

Many older sound chips had hardware support for mixing multiple streams, and so the alsa drivers for those happily allowed multiple apps to open and write to the /dev/snd/whatever device. Life was good and people got used to doing it this way.

Nowadays (since like 2000 lol), sound chips generally expect a single pre-mixed stream. So the sound device for those is exclusive open. The libalsa devs made it possible to have the first app to open the sound device act as the sound server for every other app that tries to open it later. But it was complicated and fragile and just a bad idea in retrospect.