I think I know what this could be...
If you look closely, there is a graphic equalizer graphic 'hued' onto the front end.
The dropped tiles match the height of the equalizer. My guess is, if you get a 'tetris' of blocks of the same colour in a certain space, the amount of height you have on your 'tetris' will be matched by the equalizer and that part of the soundtrack will play at that level - bass, singing, whatever.
If it's not that, then I'll suck my own schlong.