This paper proposes a music audio editing system that enables source-wise editing of harmonic instrument mixtures without explicit source separation. It builds on our previously proposed score-informed method for estimating source-wise synthesis parameters, i.e., time-varying controls used to synthesize each source, such as fundamental frequency and loudness. The method directly estimates these parameters from a mixture signal and the corresponding musical score in an analysis-by-synthesis framework. Using the estimated parameters, the proposed system allows users to edit individual sources by modifying note sequences and instrument types, and then re-synthesizes the edited mixture. Through demonstrations on two-instrument mixtures, we show that the system supports note-level phrasing modification and instrument conversion of selected sources.