Ubuntu – 18.04 LTS – What was wrong with the December 11 pulseaudio update

pulseaudio --version

This should give you an output something like this:

pulseaudio 0.9.21-63-gd3efa-dirty

To get the version of the Debian package you have installed, use:

dpkg -l pulseaudio

which will give output like this:

||/ Name             Version          Description
+++-================-================-================================================
ii  pulseaudio       1:0.9.22~0.9.21+ PulseAudio sound server

There is a nice (technical) discussion here :

http://www.alsa-project.org/~tiwai/lk2k/lk2k.html

What's Sequencer?

The role of sequencer =
Delivers events at the right time (sequence) to the right destination (device).

So this has to do with how the kernel manages your hardware , in this case your sound card. Some sound cards (commonly Intel ones) need real time sqeuencing.

The Arch Wiki's page on PulseAudio/Troubleshooting says:

The newer implementation of the PulseAudio sound server uses timer-based audio scheduling instead of the traditional, interrupt-driven approach.

Timer-based scheduling may expose issues in some ALSA drivers. On the other hand, other drivers might be glitchy without it on, so check to see what works on your system.