If you can hear ambient sound (music, weather, etc.) but no sound effects, this is one of the possible fixes:
Right click on Speakers > Sounds > Playback tab > right click on Speakers > Configure Speakers > select Stereo > Next > Next > OK.
In game configuration data and local macros location: PlayOnline\SquareEnix\FINAL FANTASY XI\USER.
Official screenshots location: PlayOnline\SquareEnix\PlayOnlineViewer\pub\home01\open\ScreenShots\FinalFantasyXI.
Directly by following steps from the loader when you launch it (third option).
Username (3-15) (as displayed on the loader for now): less than 3 characters and more than 15 characters works.
Password (6-15) (as displayed on the loader for now): less than 6 characters works. More than 15 characters works when you register BUT note that typing the first 15 characters when you're trying to log in will let you in.