We have found in testing that some audio devices - it seems HDMI audio outputs are particularly susceptible to this - may not appear in KDE’s notification panel audio applet or main audio device settings screen at all at first, giving the impression they are broken or unsupported. In fact, they will work if assigned a profile.
Bugzilla report: #2011231
From the audio settings page, click the Configure… button, and you should see the device(s) shown there, with no profile assigned to them. Once you select an appropriate profile, they should then be visible in the main audio settings page and the panel applet.