SongBook+ can play standard MIDI files, but it does not have a built-in synthesizer to produce sound from them.
To produce sound from MIDI data, you need either an external hardware connected via MIDI (such as a keyboard or a sound module) or a synthesizer app from another developer that runs in the background and receives signals from SongBook via “Virtual MIDI.”
One possible app that can be used to produce sounds directly on the iPad is, for example, “bismark bs-16i.” If this app is running in the background with “Virtual MIDI” enabled, you will hear the music directly on the iPad without external hardware.