Are any of the participants using Firefox by chance? There are known issues being worked out ¹, some in collaboration with Firefox itself ², to improve performance and compatibility.

For now, it seems the best audio/video quality can be achieved by all participants using a Chromium-based browser such as Chrome and Edge.

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¹ 100% support for Firefox (and other non-Chrome browsers) - https://github.com/jitsi/jitsi-meet/issues/4758

² Bugzilla keyword jitsi-meet - https://bugzilla.mozilla.org/buglist.cgi?status_whiteboard_t...