> Before PulseAudio became the standard, many distros used ESD, the Enlightened Sound Daemon, which came out of the Enlightenment project, best known for its desktop.
Also ARTS (KDE).
> You can play, or record, sound without a sound server, but if you don't have one, the current program that is playing sound owns the audio device: it has complete and exclusive control over it, meaning that the operating system can't mix sources.
Not really true. Long before PulseAudio came along, nearly all sound cards allowed multiple simultaneous streams. Those that didn't were supported by "dmix" in ALSA, which is automatically loaded for the default sound device these days, though sadly not for additional devices (like the USB headset you plugged-in), still requiring a bit of a configuration mess.
PulseAudio is a culprit here, too. It locks the sound device (even while nothing is being played) so those apps trying to output to ALSA can't access the sound card. PipeWire is supposed to fix all this, natively compatible with applications outputting to ALSA and PulseAudio, simultaneously. Here's hoping.