Further information :

Undub (Version 1.13) by acediez
Japanese opening song restored
Japanese story videos with English subtitles.
In-game X's and Zero's English voices replaced with japanese ones.
Restored japanese voice at the start of each boss battle (Credits: DarkSamus993)
Restored japanese voices DURING boss battles (even though they weren't dubbed, they did cut some of them).
Japanese ending song restored.