Does Freebsd use pulseaudio?

The default is a variant of the Open Sound System. FreeBSD's sound(4) implements its API and it provides the audio devices (/dev/dsp*, /dev/mixer*, ...).

PulseAudio is yet another software abstraction above them.

Support for it can be compiled into a number of ports by enabling the PULSEAUDIO port option globally. Some ports enable PulseAudio support by default or have a hard dependency on it. So effectively yes, FreeBSD uses PulseAudio.
Can I change the shared memory for OSS?
On linux, with pulse audio I would change
shm-size-bytes in /etc/pulse/daemon.conf and /etc/pulse/client.conf to 8mb

Do you know how I would do that in OSS?