Is it the horn playing out of tune by any chance? The NotePerformer horns still sound 1/4 flat (or more) in the Wagner Götterdämmerung score from this thread. I tried changing the sample rate; I also deleted these bars (5-7) and re-entered them, still really flat. I can click on the horn notes and they sound fine, but when I hit play (either full orchestra or just with horn selected), they are incredibly flat.