I think it's just the normal AddPlaySound() that's already in flyff they just inserted it in the OnInitialUpdate() probably with a switch statement for which MMI it is at least that's how I would code it.
In character.inc there's something like
Code:
SetMusic( BGM_NPC_SOLEMN2);
this may be it if it's not then it will play the sound but cancel the music that is currently playing if you search for SetMusic it only appears in project.cpp you could try to do something like
Code:
if( script.Token == "SetNPCVoice" )
{
script.GetToken();
AddPlaySound( script.GetNumber() );
}
or what ever you feel you can accomplish with your skill level you have a few options now.