I assume you are accessing the music files while connected to the same local network as the My Cloud.
If so how are the apps connecting to the My Cloud? Using DLNA, iTunes, or as a File Manager? Check the app to ensure it supports the media files you are using.
You may also need to add metadata tags to the music files in some (most) cases. There are any number of metadata editors for music files including MP3Tag and MediaMonkey.
The following may also have relevant information if accessing the DLNA media server on the My Cloud.