The download button opens the iTunes App Store, where you
may continue the download process. You must have iTunes installed with an
active iTunes account in order to download and install the software. This
download may not be available in some countries.
From Steevchik Dev Projects:
Learn My Part is designed primarily to help choir members learn their individual voice parts. There are some sample MIDI files pre-loaded but you can add your own files to the app through iTunes file sharing, or opening midi file attachments from Mail or Safari on the iPhone (detailed instructions on how to do this are in the built-in help).The app provides various ways to play MIDI files, giving the user the ability to play just their part on its own, their part highlighted with others playing quietly, or just the other parts and not theirs. Or they can do any custom mix of the tracks they like.It allows the playback speed to be adjusted slower or faster, sub-sections to be marked and repeated over and over again, and transposition.The user can choose to play each track using piano or organ sound.It can also display PDF Scores, allowing a score to be viewed while listening to a MIDI file being played back.
Updated UI to iOS 7 look and feel and added transpose functionality. Fixed a bug where if you have a large number of MIDI files loaded into the app it would sometimes open the wrong ones, and also a bug where audio would stop playing in some circumstances, such as after phone calls were received.