This problem is usually due to insufficient audio file tagging (or metadata). UPnP products such as our Network Streamers use this file 'metadata' to locate files within music libraries.
Each audio file has metadata embedded within it - this is information about the file (such as the artist, genre and album), that is usually embedded into the file at the time the file was created. This metadata is also where the album art is contained.
If the metadata is blank (which it will be if you are getting 'unknown' results) then it cannot be displayed or used to search a music library. This means you need to ensure the metadata (tags) is entered correctly to ensure successful browsing.
Another option available is to browse by folder structure instead. Many UPnP servers and our Connect app also offer a 'search by FOLDER' option, which mirrors the folder layout (rather than searching by metadata) of the music folder on the server. To access this, instead of selecting 'album' or 'artist' from within the UPnP server menu, select 'Folder' instead. This will then display the library according to your folder/file layout, instead of how your metadata may (or may not!) be organised.
Finally, if you are using .WAV as your format (CODEC) then we would generally advise against this as WAV files do not support metadata and so will default the artist and album as 'unknown'.
FLAC, AIFF or MP3 are better alternatives to get around this issue as they all support metadata.
You can use many utilities like Media Monkey/Asset/Audacity/JRiver to lookup or drag metadata/ID3 tags into all of your files.
To find out more about metadata, please read this article here.