doom2 - level numbers-lump names?

How do I find out which lump name=level music?

They all start with the letter D, followed by an underscore, and then the level they correspond to (in DOOM) or the abbreviated track title (in DOOM II.) The music for intermissions and whatnot is named after its function.

The order of the lumps in the iwad (that will also be the order they are displayed in XWE, unless you ask it to sort them into some other order) matches the order of the levels, starting with D_RUNNIN for map01 and up to D_ULTIMA for map32, followed by three more for the intermission screens, game start and stats screens music.