PulseAudio is cross-platform.. For FreeBSD in general, it uses further development of Open Sound drivers, which have things like in-driver mixing that Linux drivers under Alsa even lack.
> things like in-driver mixing that Linux drivers under Alsa even lack.
Uh?
FreeBSD's sound drivers include software mixing, so that if there are multiple things that want to use a sound card, they don't block each other. Alsa's sound channels are blocking, which means that only one program at a time can use the sound device, which is what causes the entire tower atop it to form.
Nope, Alsa has offered dmix for years now.
Which is not done in the ALSA driver, but the ALSA user-space library. In contrast to that FreeBSD's OSS drivers do the mixing in kernel-space.