Thanks for taking the time to play the game :)
I agree that the music would need to change more (for instance, when going to another act), because the main theme gets quickly repetitive.
However it is difficult to achieve this, because the music needs to react to 4 different variables, which made the number of combinations grow quicky (9 rythm/chord tracks, 6 lead tracks for a total of 45 combinations that need to blend together). Hence it takes a long time to come up with something where you can have multiple types of variation, that need to go together in some way. Not being a great composer does not help also, haha!
But it would be a cool exercice to find new themes and expand the game's music!