For this tutorial, we will be changing the battle music for generic trainers depending on the map the player is on.
 
In `GetBattleBGM` in [pokemon.c](https://github.com/pret/pokeemerald/blob/master/src/pokemon.c), this is where the magic happens.
 
First, we need to define the region map sections constants at the top of the file:
 
```diff
...
#include “constants/trainers.h”
+ #include ”constants/region_map_sections.h“
```
 
Next, go to `GetBattleBGM` which should look like this:
 
```c
 
u16 GetBattleBGM(void)
{
    if (gBattleTypeFlags & BATTLE_TYPE_KYOGRE_GROUDON)
        return MUS_VS_KYOGRE_GROUDON;
    else if (gBattleTypeFlags & BATTLE_TYPE_REGI)
        return MUS_VS_REGI;
    else if (gBattleTypeFlags & (BATTLE_TYPE_LINK | BATTLE_TYPE_x2000000))
        return MUS_VS_TRAINER;
    else if (gBattleTypeFlags & BATTLE_TYPE_TRAINER)
    {
        u8 trainerClass;
 
        if (gBattleTypeFlags & BATTLE_TYPE_FRONTIER)
            trainerClass = GetFrontierOpponentClass(gTrainerBattleOpponent_A);
        else if (gBattleTypeFlags & BATTLE_TYPE_TRAINER_HILL)
            trainerClass = TRAINER_CLASS_EXPERT;
        else
            trainerClass = gTrainers[gTrainerBattleOpponent_A].trainerClass;
 
        switch (trainerClass)
        {
        case TRAINER_CLASS_AQUA_LEADER:
        case TRAINER_CLASS_MAGMA_LEADER:
            return MUS_VS_AQUA_MAGMA_LEADER;
        case TRAINER_CLASS_TEAM_AQUA:
        case TRAINER_CLASS_TEAM_MAGMA:
        case TRAINER_CLASS_AQUA_ADMIN:
        case TRAINER_CLASS_MAGMA_ADMIN:
            return MUS_VS_AQUA_MAGMA;
        case TRAINER_CLASS_LEADER:
            return MUS_VS_GYM_LEADER;
        case TRAINER_CLASS_CHAMPION:
            return MUS_VS_CHAMPION;
        case TRAINER_CLASS_PKMN_TRAINER_3:
            if (gBattleTypeFlags & BATTLE_TYPE_FRONTIER)
                return MUS_VS_RIVAL;
            if (!StringCompare(gTrainers[gTrainerBattleOpponent_A].trainerName, gText_BattleWallyName))
                return MUS_VS_TRAINER;
            return MUS_VS_RIVAL;
        case TRAINER_CLASS_ELITE_FOUR:
            return MUS_VS_ELITE_FOUR;
        case TRAINER_CLASS_SALON_MAIDEN:
        case TRAINER_CLASS_DOME_ACE:
        case TRAINER_CLASS_PALACE_MAVEN:
        case TRAINER_CLASS_ARENA_TYCOON:
        case TRAINER_CLASS_FACTORY_HEAD:
        case TRAINER_CLASS_PIKE_QUEEN:
        case TRAINER_CLASS_PYRAMID_KING:
            return MUS_VS_FRONTIER_BRAIN;
        default:
            return MUS_VS_TRAINER;
        }
    }
    else
        return MUS_VS_WILD;
}
```
On `default` under the switch statement for trainer classes is where the normal trainer battle music plays for trainer classes that don’t have special music. It’s here that we’ll be adding in a check for the map section the player is on.
 
For the sake of this tutorial, we will be making trainers on Route 102 map play `MUS_RG_VS_TRAINER` which is the standard trainer battle music for Pokémon FireRed and LeafGreen.
 
Here’s how that would look:
 
```diff
...
- default:
-   return MUS_VS_TRAINER;
 
...
+ default:
+   if (gMapHeader.regionMapSectionId == MAPSEC_ROUTE_102)
+     return MUS_RG_VS_TRAINER;
+   else
+     return MUS_VS_TRAINER;
```

`region_map_sections.h` needs to be included because without it, we won’t be able to use `MAPSEC_*` defines with `*` replacing the map name.
 
If I didn’t mess anything up, that should work correctly.