Locate the SPW files.
sound/win/se/se021:
se021001 = flute
se021002 = harp
Copy these files to somewhere to work on.
Load Ximc
Hit SE on the bottom, then SPW > WAV and convert the files above to wav files.
Find a wav file of some music (can just rip from youtube with ripping websites), load that into audacity.
Load one of the XI sound files into audacity to compare length, cut the music down to be about the same length as the FFXI music.
Mute or delete the XI sound file so only the new one plays, save the wav out as wav (microsoft) signed 16 bit PCM.
Load Ximc
Hit WAV > SPW and convert your wav to an SPW (note, it will not save it as the same file name but a generic spw file name like se001 or whatever and it will always overwrite)
Put that new spw in place of the file in the correct folder, log all the way out and back in. The sound file is stored in memory so it won't change until you log all the way out and back in again.
These don't work with XIpivot since they load before the addon.