add music to a level? you can put custom music in a subfolder of your level folder, either called ambiance or music (ambiance is repeated, music plays once). it would have to be in .ogg format and of the name ambiX.ogg respectively songX.ogg, where X has to be a number between 1 and 255, if I recall correctly. you can also use the KS Manager for that and refer to a bunch of topics findable with the [search], I think, also the manual.
chose the song/ambiance sound to play on the respective screen by selecting their number in the editor. I'm sure you can figure it out.