Don't worry, there is a solution and that solution is called VLC.
What you need to do is install VLC on your iPhone (or any other iDevices) from the Apple Store. The app is free and does handle music and videos.
Once installed, launch VLC on your iPhone and look to the left, you can enable file sharing via WIFI to add media files...
Then simply open your browser on your Ubuntu computer and browse to the indicated URL as shown in VLC on your iPhone... Once you see the VLC page in your browser, simple add music (or movies) to the VLC database.
Files can be manage directly from the VLC app if you want to remove them. Since VLC is already a beautiful app and does actually play many formats, you can just ditch the stock music app and rely completely on VLC for your music listening on your iPhone.
Of course, this tip also works on any computer able to support a browser. It probably works with Android devices as they are able to browse for any files.