I spent a few minutes on it. Interesting idea and appealing visuals, room for improvement, but considerable built-in limitations.

I think on the pitch-matching exercises and others the app should briefly sound the starting note at first, before the bar hits the line, so that users are at least in the ballpark and not scrambling to find the pitch after the exercise has started.

The major annoyance I had was in the Puer Natus exercise. The words are sideways on the bars (I turned my phone to read them, which is fine), but then the pitch height is inverted with respect to the text's orientation. Very confusing and resulted in me flubbing an antiphon I could otherwise sing from memory. Either the text or the pitch height presentation needs to be flipped around.

I think as a limited component of beginner training, or for a bit of fun, it could be neat. One would want to move on from it though because the singing style it reinforces is (necessarily) quite mechanical and anti-rhetorical, and it does nothing to teach interpretation or sight singing of square notation. If I were you I would not make this a big project, since the learning application is limited.

I probably did have the screen orientation locked. My mistake.

With the movable pitch - this is planned and the idea is this: when the 'pitch adjustment' is toggled on, the piece is transposed so that the dominant falls in the center of the vocal range (that was measured during the onboarding or later).