ZyXEL Communications NSA325 Support Note page 10

You can select one category in the list to play the music.
Figure 33. Play the music by selecting the category.
Just click on the song to start playing the music.
Figure 34. Music Folder.
Figure 35. Music Files.
If you find that the media files can't display correctly, please check the status
in Applications > Media Server, or click "Rebuild Database". Usually, you will
some time to see the photos displayed.