It varies from place to place on different phone manufacturers, but is usually in
/Android/data/com.elf.koalasampler/files/songs
On Android 11 and above this folder may not be accessible from your phone, but it should be from your computer – on Mac, Google make an app called Android File Transfer which will let you access this folder. On windows, you should be able to access it in Explorer.
If you don’t have access to a computer to see this folder, it’s possible to access it with an app like “Solid Explorer” or “FX File Explorer”