This hack changes the music track that plays during the final boss fight from the Double Dragon Theme to the unused Duke's Theme.
It also changes the text of SOUND TEST & EFECT TEST to MUSIC TEST & SOUND TEST in OPTION MODE as well.
The 0xED hex editor was used to change the music track and the text.
Return of Double Dragon (Japan).sfc
MD5: EF37E657E2BB1FBFF7C1C09A184028B5
SHA-1: 5F03E04CE7BCA88F8C2C11934056A3A67BFB319B
SHA-256: B61ADDB0ABD36EBD29E5C2988AE642B174BFEC18A899CFE40866000DC934F658