You can change the instrument sound of the melody or harmony by touching the info button for the chords or the one for the staff. Change the tempo by touching the metronome button.
The Options button at the top of the screen lets you choose the length of the idea in measures, and also the degree of rhythmic complexity. "Simple" rhythm means just eighths, quarters and halves when in meters like 2/4 or 4/4.(In meters like 6/8 and 9/8 a simple rhythm would include dotted notes.) "More complex" means there will sometimes be dotted notes in meters like 2/4 or 4/4. "Complex" means you'll start to see 16th notes appearing in the tune. These remain unpredictable, though - some tunes will seem more complex than others regardless.
In addition to those options, you can also choose the clef, key and meter for each idea. When you choose a new key or clef, the current idea will be transposed. Subsequent ideas will be generated in a range appropriate for the new clef and using the key signature for the chosen key. Changes to the meter will, similarly, be applied to all subsequent ideas.
You can experiment with the chords suggested by Musica using the chord tool. Touch the Chord tool or double-tap on any chord to view the chord tools palette. Here you can replace one chord with another, alter the chord's quality, and even change its "playpattern." If you have selected several chords at once your choice will apply to all that are selected.
Some of the ideas generated by this system were so charming that we saved them for posterity. You can hear one of our favorites on the Songworks information page at Ars Nova (Songworks uses the same tune generator). But you never know when you might find a better one.