When was that? I think my first Linux distribution was Ubuntu 8.04 and fairly sure it shipped with PulseAudio which in mind always been able to play audio from multiple sources at the same time, maybe I misremember?
Upsite: Highest quality playback.
Downside: Only one process could play audio at a time.
The most common solution at the time was PulseAudio, which was so bad it usually was better to just use direct ALSA and live with the idiotic one-at-a-time limitation.
Thankfully Pipewire seems to actually work reliably so I guess that's at least one thing ticked off the Year of the Linux Desktop checklist.
Which is why the whole "we must use pulseaudio even if it's terrible and has awful standards that blast volume or multiple streams won't work!" was so weird… everybody who tried knew that just removing pulseaudio the multiple streams kept working :)
So only those who never applied the scientific method kept insisting that without PA it was not possible to do that.
Plus of course, initially you had to regularly run killall -9 pulseaudio to fix the sound. All in a moment when ALSA with dmix worked just fine.
Sometimes I think fedora and ubuntu are trying to hinder linux as mainstream desktop.
Around 2000 I was only able to play sound from different apps because my soundcard exposed two sound devices /dev/dsp0 and /dev/dsp1
I still don’t know what purpose pulseaudio serves, other than adding latency and making stuff less reliable.
PipeWire is better, but it turns out you can just use OSS under freebsd these days, and everything just works, but with lower latency.
If you have some sort of potato sound card that can’t mix output channels in hardware, note that OSS added sw mixing by 2007 (with support for 16 channels by default).
OpenBSD still present raw audio devices, but they have sndio which provides a more helpful interface for applications including resampling (not the best algorithms there, according to them).
