Hi, sound guy and also partial level design guy here :) glad you enjoyed the game so much!
The way that the music works is that it's technically divided into 8 different layers, with the first 6 being the ones that come in during the main gameplay and make up different elements of the song, and then layer 7 being a stripped down version of the piano, and layer 8 finally being a loop of rushing water that comes in when the timer's almost up. When the game is in the menu, just layers 1 and 7 play. In-game, another layer gets added incrementally every 10 rooms, so by the time you're at the evil plants, all layers kick in, including an accordion, some extra percussion, and layered clapping.
Thanks so much for playing and leaving such a glowing review! Everyone on the team was happy to see it :)