That song isn't on your device...
Do you see this warning message when you first set up OpenSongApp or change your storage location?
The issue is caused by a storage permission issue that does not allow the app to properly save permissions to the location chosen. The app can see that there are songs there, but cannot open them.
Here's how it is caused and how to fix it...
When you start the app and are asked to choose a storage location, this has to be done using Android's built in storage access framework. You'll see a screen similar to the following screenshot.
Note that Android will often give you a shortcut location to your Downloads folder when you pull out the side menu and by default this is the one Android suggests - DO NOT USE THIS AS THIS IS USUALLY THE PROBLEM!!!. This shortcut gives you one time access only - in other words, the app cannot read/write to this shortcut location once you have created the initial folders.
You need to choose your device's internal storage. If you can't see this from the pull out menu, you can switch it on using the three dos at the top right of the file chooser. Opening the pull out menu to the left should then show your storage. THIS IS THE LOCATION YOU HAVE TO SELECT,
DO NOT USE THIS LOCATION!!!
This is a shortcut that you cannot set as permanent storage.
Make sure you have chosen to show internal storage
You should now be able to see a link to your storage (in my case 'Gareth Galaxy S9'). This is the one you should choose.
Once you choose your device's storage, you can then browse to an appropriate storage location, either in the root (the first location you are shown when you click on it), or even to the actual Downloads folder if that's where you'd prefer. Regardless of the storage location you choose, you should then click on the 'Allow access' button and Android will show an alert to what you are giving the app permission to access. This should include a reference to your device's storage as shown below.
An overview of my storage. I could choose this first location, or browse to the documents or Downloads folder
After then opening the Downloads folder, I can choose this location by clicking on the 'Allow access' button
You should then see an overview screen like this. Notice that it shows that in my case I've chosen the 'Downloads' folder located on 'Gareth Galaxy S9'. This will work.
You don't have to use the Downloads folder. You can use most folders here with the exception of the Android and data folders (these are system folders).
If I'd chosen the root folder, I'd see the screen below. Note that it still names my device's storage.
An example of badly chosen storage
If you had simply clicked on the Downloads link in the storage menu that Android gave you at the start, you would get a notification that you are allowing access to Downloads on Downloads. This won't work and will give you the error that songs cannot be found / may have been deleted.
Don't use this one - purely an example to show the problem
Note the issue - 'Downloads in downloads'! This causes the problem described. The app gets permission when you use the link the first time, which will create the folders, but then when the app tries to go back, it no longer has permission as Android gives the location a temporary name that changes.