I can't use removable storage...
A few users are starting to experience problems with external storage not being available. This is due to updates to Android security (nothing to do with me, despite criticism in the Play Store comments). I've always encouraged people to use internal/emulated storage - which I still suggest you do.Affected devices mostly seem to be Samsung ones currently (my old Galaxy Tab2 7.0 is affected) and devices running Android Lollipop or later. If you can move or copy your songs (using a file manager) and change your storage location in OpenSongApp, I'd highly recommend it. Check the internal/emulated storage location reported by OpenSongApp. This is the best place to store your files.
Some devices do allow you to format your external SD card as extended storage (i.e. not mounting it as a portable storage SD device). This allows you to increase your storage space, but setting this up for the first time will wipe the SD card. Please back up anything that is important! Using a SD card in this way will allow the app (and any other app) to use it.
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!