ALSA is just the kernel layer
Just one point about ALSA; most of it actually lives in userspace. The ~/.asoundrc file can specify complex routing and mixing, including the use of LADSPA filters. What's done is done, and pulseaudio is reasonably usable now, but IMO we'd be much better off if the work that went into Pulseaudio and jackd instead went into the ALSA userland.