There is no smooth transition between music (like if you wanted to have it so there's the slums, the garden, the rich area, and the marketplace, of a town) or waterfall/flames.
Because RPGMaker allows for multiple sound effects but not BGM or BGS.
So, perhaps might you be interested in making it so that RPGMaker can handle multiple of those for this script?
I know that might sound a bit wild, but most modern games I know can fade between one song to another or flame to flame.