Hi,
Under Debian 11 plasmashell 5.20.5 notification sounds do not work for firefox or chromium with an application like google calendar

In systemsettings > Notifications > Applications settings
Firefox-esr that is installed and defined as default web browser do not appear
Chromium appears with the mention "That application do not allow to configure notifications" and notifications do not work
Thunderbird appears with the same mention but new messages generate notifications sounds
Kmail appears and shows a "configure events" link where you can play sounds.

how can I configure a web browser to generate notifications sounds ?

You need to have plasma-browser-integration package installed, and in firefox at least you need to get https://addons.mozilla.org/en-US/firefo ... tegration/ addon. That should get you notification integration.