Notwithstanding practical limitations imposed by mobile device platforms and applications, truly captivating musical instruments may be synthesized in ways that allow musically expressive performances to be captured and rendered in real-time. In some cases, synthetic musical instruments can provide a game, grading or instructional mode in which one or more qualities of a user's performance are assessed relative to a musical score. By constantly adapting to such modes to actual performance characteristics and, in some cases, to the level of a given user musician's skill, user interactions with synthetic musical instruments can be made more engaging and may capture user interest and economic opportunities (e.g., for in-app purchase and/or social networking) over generally longer periods of time.