This is possible, with some limitations. It works on Chrome browsers only.
On macOS and Linux you can only share the audio of a tab, not the full system audio. On Windows, you can share the audio of the full desktop too, see a “share audio” checkbox at the bottom of the screen picker.
If you only want to share audio but not the tab or desktop itself, we have that coming, it will be available soon, same limitations apply.
Even with these limitations I like the Share audio feature ! (I only recently updated Jitsi and noticed the new features).
On one of my laptops, using the internal microphone and speaker, sharing desktop, the audio on Linux was still coming through the meeting. When I used headphones, the audio did not. The laptop does not have echo issues either. I wonder if anyone else has observed similar behaviour?
I guess so, similar to a person speaking. So echo cancelling is between the microphone and the sound coming out of the Jitsi session (not sound coming from the computer)? I kind of I think I understand (maybe).