The first thing I'd do is get rid of pulseaudio and access the ALSA
device directly. Even then, however, there are several places where
latency is added. You can find the ALSA buffer sizes currently uses by
examining /proc/asound/card0/pcm0p/sub0/hw_params (or whatever numbers
your system uses). The size (in bytes) of the main buffer in SoX is set
with the --buffer option. Some effects have additional internal buffers.