Play audio output as input to Zoom
We had exactly the same problem with Zoom in Linux (Ubuntu, Lubuntu and Gentoo). The solution turned out to be as follows, and does not require using PulseAudio Volume Control:
Launch the application for playing audio or video. Pause the audio/video if necessary.
Click on 'Mute' in Zoom, to mute the mic (see NOTE below).
Click on 'Share' at the middle bottom of the Zoom window, select the application (or 'Desktop') and either a) click 'Advanced' if you only want to share audio, or b) click on 'Share computer sound' and on 'Optimise Screen Sharing for Video Clip' if you want to share a video.
Click the blue 'Share' button.
Unpause/start the audio/video playing in the audio/video application.
NOTE: We find we get much better audio quality by muting the mic before sharing audio/video. After starting the audio/video stream, it is then possible to unmute the mic.
Make Zoom accept monitor as input (Ubuntu 20.04)
With the new PulseAudio version 13.99+, you can no longer select "System default" as mic in Zoom, which was a way of tricking Zoom into using a monitor as input. You have to create a virtual source:
pactl load-module module-null-sink sink_name=zoom_input sink_properties=device.description=zoom_input
pactl load-module module-remap-source master=zoom_input.monitor source_name=zoom_mic source_properties=device.description="zoom_mic"
By doing this, "zoom_mic" will show up as a input device in Zoom.
Using pavucontrol, redirect the sound from your media player to "zoom_input" and it will be played in Zoom.
See also this advanced guide to PulseAudio connections: https://github.com/toadjaune/pulseaudio-config