The ALSA project was born back when Linux was in version 2.4 — and unlike today, that version was the version for a long time. Indeed up until version 3.0, a “minor” version would just be around forever; the migration from 2.4 to 2.6 was a massive amount of work and took distributions, developers and users alike a lot of coordination. In Linux 2.4, the audio drivers were based off the OSS interface, which essentially meant you had /dev/dspX and /dev/mixerX, and you were done — most of the time mixer0 matched a number of dspX devices, and most devices would have input and output capabilities, but that’s about all you knew. Access to the device was almost always exclusive to one process, except if the soundcard had multiple hardware mixer channels, in which case you could open the device multiple times.