This problem is usually due to the audio file tagging (or metadata) being insufficient. UPnP products such as the Stream Magic 6 use file 'metadata' to browse music libraries.
Each audio file has metadata embedded into it - this is information about that file such as the artist, genre and album, that is usually embedded into the file at the time of ripping (when 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 of course it cannot be displayed or used to search a music library.
Metadata is used for browsing because if for example, if you had 1000 random tracks dumped in no particular order on a drive, there would be no way that you could possibly navigate by artist/album/etc...
However, the beauty of UPnP, is that the Stream Magic 6 looks at the categorised metadata that is made available by the UPnP server (this can be a NAS drive or UPnP server programme such as Twonky or Asset running on your computer).
This means that you can now browse those same random 1000 tracks by artist/album/genre/etc (depending on what metadata the server supports).
Of course, using the metadata also gives you added bonuses such as album art if using the Stream Magic Remote app to control the player.
This of course does mean that you must make sure the metadata (tags) are fully entered correctly because successful UPnP browsing depends so much on good file metadata. If a track is limited on metadata, then this is the reason it is not universally indexed on the UPnP server and will show up as 'unknown' or worse still, may not show up at all. If for example, the Artist and Album tags are blank, then these of course cannot be searched for.
To prevent this from happening again, you need to enter the file information into the metadata of each file, so that the UPnP server can index the files correctly. You can use many utilities like Media Monkey/Asset/Audacity/JRiver to lookup or drag metadata/ID3 tags into all of your files very easily.
Another option that will be available to you is to browse by folder structure instead. Many UPnP servers 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 browse 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. 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.