I don't use iTunes, I just use plain MP3 stored in directories, I also have FLAC files. I stored them on OneDrive and use the OneDrive application from the APP. I use the application to download them on my phone "Make the file available offline".
The OneDrive application sucks and only allow to play 1 file, there is no queue, no playlist, nothing. I am looking for a solution to read a directory of audio files on iPhone.