[SDPhantom]

Here's code for the main part of the addon that I whipped up. It uses metatables to handle falling back to a generic song table. Be sure the file containing this code loads first in your ToC.

--  Grab access to our shared addon table
local _,Addon=...;
 
--  Set up our default song table and metatable for fallback
local DefaultSongTable={};
local FallbackMeta={__index=DefaultSongTable};
 
local SongTables={Alliance={},Horde={}};--  Build faction-based song tables
for _,tbl in pairs(SongTables) do setmetatable(tbl,FallbackMeta); end-- Set our fallback metatable
SongTables.Default=DefaultSongTable;--  Register the default song table
Addon.SongTables=SongTables;--  Enable access to our song tables through the shared addon table
 
local CurrentSong;
local function PlaySongForMap(mapid)
--  Select the song table and grab the song for the map
    local song=(SongTables[UnitFactionGroup("player")] or DefaultSongTable)[mapid];
 
--  Play the new song if different or stop if there is none
    if song and song~=CurrentSong then
        PlayMusic(song);
    elseif not song then
        StopMusic();
    end
 
--  Update our saved variable
    CurrentSong=song;
end
 
local EventFrame=CreateFrame("Frame");
EventFrame:RegisterEvent("PLAYER_ENTERING_WORLD");--    Player enters from loading screen
EventFrame:RegisterEvent("ZONE_CHANGED_NEW_AREA");--    Zones changed
EventFrame:RegisterEvent("ZONE_CHANGED_INDOORS");-- Some indoor zone changing requires this
 
EventFrame:SetScript("OnEvent",function()
--  We don't want to mess around with the map while the user is looking at it, but we need our MapID, so we'll do some tricky stuff
    if WorldMapFrame:IsShown() then
--      Get a list of frames listening for map updates and disable them
        local frames={GetFramesRegisteredByEvent("WORLD_MAP_UPDATE")};
        for _,frame in ipairs(frames) do frame:UnregisterEvent("WORLD_MAP_UPDATE"); end
 
--      Save old MapID and select current zone (MapID is invalid if viewing a continent, store that too)
        local mapid,continent=GetCurrentMapAreaID(),GetCurrentMapContinent();
        SetMapToCurrentZone();
 
--      We have the zone we want, find and play our song
        PlaySongForMap(GetCurrentMapAreaID());
 
--      Restore the map and re-enable the frames so the user would never know we did anything
        if mapid>=0 then SetMapByID(mapid); else SetMapZoom(continent); end
        for _,frame in ipairs(frames) do frame:RegisterEvent("WORLD_MAP_UPDATE"); end
    else
--      User isn't looking at the map, we'll use a more straightforward approach
        SetMapToCurrentZone();
        PlaySongForMap(GetCurrentMapAreaID());
    end
end);

In your sound packs placed in other Lua files in the same addon, you can register songs like this.

--  Grab access to our shared addon table
local _,Addon=...;
 
--  Alliance and Horde have their own songs, though it's a good idea to define a default anyway
Addon.SongTable.Default[123]="Path\\To\\DefaultSong";
Addon.SongTable.Alliance[123]="Path\\To\\AllianceSong";
Addon.SongTable.Horde[123]="Path\\To\\HordeSong";
 
--  Alliance has their own song, Horde uses default
Addon.SongTable.Default[345]="Path\\To\\DefaultSong";
Addon.SongTable.Alliance[345]="Path\\To\\AllianceSong";
 
--  Horde has their own song, Alliance uses default
Addon.SongTable.Default[567]="Path\\To\\DefaultSong";
Addon.SongTable.Horde[567]="Path\\To\\HordeSong";
 
--  Both factions use the default
Addon.SongTable.Default[789]="Path\\To\\DefaultSong";

PS: The first return of UnitFactionGroup() is locale-independant, so it doesn't need to be localized.