Night Stalker, to convert mp3->wav->bgw do this:
mp3->wav
There are a number of mp3<->wav converters out there. My favorite is
Acoustica MP3 converter but it costs money. There are a number of freeware converters. Note that I havn't tried these but I quickly "researched" these using google. Try that next time, you'll get your answer quicker

. If you need help in these read the included instructions but they usually are straight forward.
LamedropXPd
http://rarewares.hydrogenaudio.org/f...pXPd-1.3.3.zip
dBpowerAMP Music Converter
http://www.dbpoweramp.com/dmc.htm
(can rip CD's aswell)
wav->bgw
Once the software has converted your mp3 to wav, use the BGW converter. Select which game track you would like to replace and press the button under WAV->BGW. Select your file and press open. Wait a bit and it should be done. The converter places the bgw file in the same directory as your wav, so if you had your wav in your FF11 directory then the converter would replace the origonal bgw file. It does not ask whethere to overwrite or not so be carefull. If the converted file is in another directory then just move it to
"X:\Program Files\PlayOnline\SquareEnix\FINAL FANTASY XI\sound\win\music\data" and overwrite the origonal youself.
Alright it can't get any more simpler then this. If no one is able to do this now then maybe it's best to leave it alone hehe.