Comment # 8 on bug 858498 from Wolfgang Bauer

The problem might also be that those files are already indexed. If that's the
case, the information from the index is shown. If they were indexed before you
installed kfilemetadata-ffmpegextractor, the index doesn't contain the duration
of course, and therefore nothing is shown.

So, try to disable indexing in "Configure Desktop" (systemsettings)->Desktop
Search, then dolphinshould display the information.

This will also delete the index, so you can enable indexing again afterwards,
your files should get reindexed then including the additional details.