Did you restart Dolphin after installing those packages?

Yes, I restarted dolphin and the whole system

Okay, unfortunately I don't know why the information isn't being displayed then - as if all appropriate plugins are installed, then it should be able to extract the audio metadata from the files. As a final test, does anything usable appear under "Information" in each file's properties?

For additional information are displayed: size, date, type, route, destination, permissions, owners, user group, but no audio is what matters to me mp3 files: artist, album, duration, track.
In a musical properties file appears in general: type, location, size, modified, accessed. Permissions

You know a file browser that displays artist, album, track length, in Manjaro?

Dolphin should be able to display that information - but the necessary Strigi plugins appear to be missing unfortunately.
You'll need to ask the Manjaro packagers which package contains the Strigi plugins responsible for multimedia metadata.