Master Levels music
From DoomWiki.org
Most versions of Master Levels for Doom II offered no new music, instead having each level playing the same track as the one it replaced. This changed with the version included in Doom + Doom II, which used UMAPINFO to assign various tracks from Doom II's IWAD to different slots while also including copies of tracks from the first game. Prior to this, Peter Lawrence (MegaSphere) put together a MIDI pack for the collection's 25th anniversary.
The file masterlevels.wad only contains the music tracks originating from Doom (D_ExMy naming scheme, plus D_VICTOR). As it is not an IWAD, the tracks from Doom II are instead loaded from doom2.wad.