I saw a slider for transposing. Don't you need two? One should change the pitch, and the other should change the notation. I tried yours, and it changed both pitch and notation at the same time. (listening to the synthetic sound)
For music with multiple parts, it would be nice to transpose everything together in the most reasonable way. This would maintain relative pitch relationships, adjust the whole thing up or down to fit ranges of the players, transpose according to how the players will read music, and avoid excessive accidentals. A few alternatives could be offered: option 1 makes the trumpet player suffer really high notes, option 2 makes oboe player suffer lots of flats, etc.
For music with multiple parts, it would be nice to transpose everything together in the most reasonable way. This would maintain relative pitch relationships, adjust the whole thing up or down to fit ranges of the players, transpose according to how the players will read music, and avoid excessive accidentals. A few alternatives could be offered: option 1 makes the trumpet player suffer really high notes, option 2 makes oboe player suffer lots of flats, etc.