The soundtrack (music) is untouched. Only voice files are swapped. The excellent orchestral score remains identical.
: This version is a full "rom-hack" that integrates the Japanese audio into the game files.
Distributing a full, pre-patched ROM (CIA/3DS) is piracy. Downloading only a patch (XDelta) is a grey area. Most modders consider it fair use if you own the original game. I do not condone piracy; this guide is for users who have legally dumped their own cartridges.
The Undub Link feature allows players to create a link between the original Fire Emblem: Gaiden and Fire Emblem Echoes: Shadows of Valentia. This link allows players to unlock special content and features, such as:
Turn on your 3DS. Boot into Luma3DS configuration (hold Select on power up). Ensure "Enable game patching" is checked. Press Start to save.