The original game packed the background music and environment sound effect locations for zones in a binary ZoneNick_sounds.eff file, text ZoneNick_sndbnk.eff file that listed the sound effects referenced in the binary file, and ZoneNick.xmi multi-song MIDI files.
With Planes of Power, they added mp3 music for some of the higher-level planes, added the list of them to mp3index.txt, and pointed music references in the appropriate planes' x_sounds.eff files to these new mp3s by using negative numbers for the song numbers.
In Legacy of Ykesha, they completely revamped the system. No more xmi music files and no more eff files for new zones. Zones newly created now used a text-based .emt file that listed the locations and characteristics of background music and environment sounds for zones.
No documentation was ever released on either format, but a request from Grimluck on figuring out how to customize music in the zones led me on a journey to reverse engineer them.
The result is a tool that can convert the old-style .eff files for Original through PoP zones into the editable .emt file that can be customized at will. If the client finds a .emt file for a zone, it'll ignore any old .eff files that are hanging around for it.
The field definitions in the comma-delimited .emt file are also included in any converted files, for easy human reference while editing.
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The source code for the command-line version is here:
https://github.com/Shendare/Eff2Emt
The source code for the GUI version in .Net 3.5 C# is here:
https://github.com/Shendare/Eff2EmtGUI
Compiled binaries can be found here:
http://www.shendare.com/EQ/Emu/Eff2Emt/
Current Screenshot (GUI version):
Screenshot Mirror (in case my site goes down for some reason):
http://i.imgur.com/6Dbpmhj.png