The "Installation for Linux" page for the Linux Client needs updating because of the additional dependency added under jamulussoftware/jamulus#2721, which fixed jamulussoftware/jamulus#2720.
For Ubuntu only, prior to the "Download Jamulus" step, we need to add:
You will need to enable the "universe" Ubuntu repository before installing. Please see one of the following two posts:
(Note: Debian does not have "universe", only "main", "contrib" and "non-free" and libqt5multimedia5 is in "main", so there should not be a problem.)