[kbolino]

This assumes there is an OS-managed software mixer sitting in the middle of all audio streams between programs and devices. Historically, that wasn't the case, because it would introduce a lot of latency and jitter in the audio. I believe it is still possible for a program to get exclusive access to an audio output device on Windows (WASAPI) and Linux (ALSA).

[dahauns]

Historically, true, but nowadays it's pretty much standard for all the big OS.

Being able to get exclusive access/bypass the system via certain means (ASIO would be another) doesn't make it go away.