To change your music that plays in Aika a little know how i required, if you don't understand this guide then don't attempt it.
-First i suggest you back up your files in the bgm directory in case you want to restore default sounds without doing a fresh install.
The right folder is bgm in all aika client's
-Second is the good part, to replace a area with your desired mp3, take your mp3 and rename to the name associated with the area you wish to change, a list can be found below.
This List is not complete, please feel free to help with the edits.
AirGarden - kynari aviary's
Basilan - Cirrugor
BattleField - BattleField
BattleFieldLobby - BattleFieldLobby
BlizzardHurricane - Air Carrack
BrightWood - DarkRane, Basilica Giovannis
BrokenRange - Ruins of Algon, Mt. Hessian
DesertWind - Tabazra Desert
DespairWood - Ashwood
DisusedMine - Lost Mines
Efesso - Epheso
Gentile - Rhawn Crossroads
PhantomShip - Marauder Hold
Regenschein - Regenshein
RestrictedArea - Lost Mines
Screaming - Zant Final boss Room
Seridunn - kynari aviary's final boss
TalkToMe - Music when you talk to your pran.
TempleOfDusk -Zantorian Citadel
TombOfShips - Celestine Caverns
Verband - Verband , Zantorian Park, PD, Glacial cave
WhiteHall - Amarkand