Starting with SongBook+ 3.4.0, a new audio player is used that allows the features of transposing audio files and an equalizer.
This new audio player may cause stability issues on older, less performant iPad models.
For stability reasons, SongBook+ again uses the older audio player on older iPads (with iOS 9), and therefore the features to transpose audio files and the equalizer are (per default) no longer possible on these devices.
But if you still want to use the newer audio player (and the features to transpose audio files or the equalizer), it is still possible. But you do this "at your own risk" and accept possible stability issues. To do that, open the settings of SongBook+ and select "Others", and write the text "NotOldAudioPlayer" into the field "Beta test flags".