You have a Dropbox account? And in the Dropbox are from time to MP3 files that you want to consume on the go? We wanted this in the summer of 2014. At that time, we have worked extensively with the Universal applications Cloud Beats, Boxytunes and Tunebox that promised all to offer a solution for the task initially described.
Forget the candidates and grab the free iPhone Download Jukebox. Ad-free and with no in-app purchases, the hobby project cares about, painless to transfer new MP3 files that have landed in your mailbox or on a perfectly ordinary web download on your hard drive to the iPhone. Without iTunes.
You do not do more, than put the audio files to store in the personal Dropbox folder. Then wait a short moment for the synchronization and start the jukebox app. Jukebox scans your Dropbox for existing MP3 files, indicates them in a clear list and allows you to transfer the desired tracks with a tap to the iPhone.
The loaded songs, podcast files or Web downloads can be stored in playlists, can be if needed deleted and are played in jukebox own audio player.
A new version, where the considered folders can be selected and deselected by a tap, is waiting for its release by Apple.