Music Speed Changer allows you to access tracks on the device and on cloud storage through the Android Document Picker. This method uses the devices internal file manager and may allow you to find files that are not visible in the Library.
Open the App Main Menu

Select System picker

If your device takes you directly to drive, you can access Drive by selecting your remote account. From here you can download the files you wish to edit

Or you many need to open the system menu to get to your cloud account

Select your Drive Account. From here you can download the files you wish to edit

Tracks selected using this method will be available in the Library after Music Speed Changer imports them.