Put your own music files here (.mp3 or .ogg). The game picks them by name:

  town.mp3      - Aldoria town
  field_1.mp3   - Pudding Meadow
  field_2.mp3   - Leafy Grove
  field_3.mp3   - Buzzing Hills
  field_4.mp3   - Fiddler Fields
  field.mp3     - fallback for any field without its own file
  boss.mp3      - while an MVP boss is chasing you
  forest_1.mp3 .. forest_4.mp3, forest.mp3 - Whisperwood Forest (falls back to field.mp3)
  desert_1.mp3 .. desert_4.mp3, desert.mp3 - Sunscorch Dunes (falls back to field.mp3)
  dungeon_1.mp3 .. dungeon_3.mp3, dungeon.mp3 - Hollowroot Caverns (falls back to field.mp3)
  town_2.mp3    - Hanamura, the bamboo village (falls back to town.mp3)
  field_5.mp3   - Hanamura Outskirts (falls back to field.mp3)
  town_3.mp3 .. town_6.mp3 - Sugarplum, Brightwater, Snowbell, Toyholm (fall back to town.mp3)
  field_6.mp3 .. field_14.mp3, field_ga.mp3 .. field_gf.mp3 - the other fields (fall back to field.mp3)
  dungeon_4.mp3 .. dungeon_7.mp3, dungeon_ga.mp3 .. dungeon_gf.mp3 - the other dungeons (dungeon.mp3, then field.mp3)

Missing files are fine: that area just stays quiet (or uses field.mp3).
In game: M = mute, [ and ] = volume.