Indicating what should be played in a piece of music with a music document, including: displaying a part of the music document when a user plays the piece; receiving a real-time audio signal of the playing; automatically determining a playing position within the piece of music based on the real-time audio signal; automatically scrolling the music document on a display depending on the playing position; estimating at least the following from the real-time audio signal: activity; tonality; and tempo used in automatically determining the playing position determined from playing speed of the user. The estimating of the activity includes detecting whether the user is producing any sounding notes. The estimating of the tonality is based on an array of chord models that represent different chords that appear in the music document and allow calculating the probability that the corresponding chord is being played in various real-time audio signal segments.