The Music in Diablo II was composed by Matt Uelmen and is composed of over 30 different songs for various parts of the game.
All the music can be downloaded here:
Diablo II Music, Cinematics and Speech.
Locations where it plays
Main menu if the expansion isn't installed
Black Marsh, Blood Moor, Burial Grounds, Cold Plains, Dark Wood, Tamoe Highland
Original track that was later reworked to the "wild" track used in Act I.
Chaos Tristram, Tristram
An extended version of the same tune found in Diablo I.
Cave, Den of Evil, Hole, Pit, Underground Passage
Crypt, Forgotten Tower (Tower Cellar), Matron's Den, Mausoleum
Barracks, Catacombs, Cathedral, Inner Cloister, Jail, Monastery Gate, Outer Cloister
Dry Hills, Far Oasis, Lost City, Rocky Waste, Valley of Snakes
Discarded by Matt Uelmen because it lacked harmonic development.
Canyon of the Magi, Forgotten Sands
Ancient Tunnels, Lut Gholein Sewers
Harem, Palace Cellar
Claw Viper Temple, Halls of the Dead, Stony Tomb, Tal Rasha's Chamber, Tal Rasha's Tomb
Flayer Jungle, Great Marsh, Spider Forest
Kurast Bazaar, Kurast Causeway, Lower Kurast, Travincal, Upper Kurast
Disused Fane, Disused Reliquary, Durance of Hate, Forgotten Reliquary, Forgotten Temple, Kurast Sewers, Ruined Fane, Ruined Temple
Arachnid Lair, Spider Cavern
Abaddon, City of the Damned, Infernal Pit, Furnace of Pain, Outer Steppes, Pit of Acheron, Plains of Despair, River of Flame
Chaos Sanctuary, Secret Cow Level
Main menu if the expansion is installed,
Throne of Destruction*, The Worldstone Chamber*
Arreat Plateau, Arreat Summit, Bloody Foothills, Frigid Highlands, Frozen Tundra, Nihlathak's Temple
The Ancients' Way, Crystalline Passage, Drifter Cavern, Frozen River, Glacial Trail, Icy Cellar
Halls of Anguish, Halls of Pain, Halls of Vaught
Throne of Destruction*, Worldstone Keep, The Worldstone Chamber*
* Both the Throne of Destruction and The Worldstone Chamber start out by playing introedit once, after which they continue with baal.