I can't use removable storage...

If you are running Android Lollipop or later, this is no longer an issue as this was fixed in V5.0.0. KitKat users will still not be able to use the external storage card unfortunately. Older devices are now no longer supported. These issues were caused by enforced Android security permissions. Technically without these permissions, apps could do what they liked with files on your storage, now you have to explicitly define the folder that the app can use - i.e. the OpenSong/ folder.

internal storage

I've played around with a way of changing how the storage can be accessed (e.g. using Android's StorageAccessFramework), but with limited success. I can just about make it work in the app, but with severe limitations including speed, but currently Dropsync and Dropbox can't use the affected device storage either, so that would create other problems. I'm unlikely to be able to fix this, so USE INTERNAL STORAGE!