I already found a solution, but I'm adding more to the topic, in case anyone finds this, since the other topic is already locked.
You don't need to reboot after making the change to /etc/pulse/default.pa, where you need to comment out the line that loads the module named module-role-cork.
You can issue the following command, without even using sudo, to unload it from the running instance of PulseAudio, so it takes effect immediately:
pactl unload-module module-role-cork
It's supposed to "stick a cork" in your streaming audio or video, whenever a communications audio stream starts playing. Unfortunately, it seems that all system notification sounds have the same stream identification, so they mute everything as well. What a shame.