First off, you need a dump of your game. More info can be found here.

Go into the folder where you store your game and it should be in
[game]\content\stream\bgm
The files for the NLA themes for day and night are x63_00.adx and x64_00.adx, respectively.

Alright, to start off with, the files for the music in the game is in ADX format and as such the music you want in the game need to be converted.

This program can convert your MP3 files to ADX.

Alright. Now that you've successfully converted and renamed your files you simply replace the ones that are in the game. And remember to store the original files in a place where you can remember it. Just in case if you want to add them back.

Also, if you want to change the flying theme, the name of that file is x79_00.adx

Enjoy the game.

 

[RealityNinja]

You can use cafiine to have the name of all music files... And patch the game... on the disc/eShop version. https://github.com/NWPlayer123/Cafiine-5.5.X
No need to use loadiine! (and spend 12 hours to dump the game with ddd XD)

But..

The NLA music is just amazing, why do you want to replace it?

edit: but thanks for writing the tuto...

 

[SweetAphrodite]

You can use cafiine to have the name of all music files... And patch the game... on the disc/eShop version. https://github.com/NWPlayer123/Cafiine-5.5.X
No need to use loadiine! (and spend 12 hours to dump the game with ddd XD)

Didn't know that. Don't I feel like an idiot.

But..
The NLA music is just amazing, why do you want to replace it?

It's really loud and gets real repetitive fast. I didn't like it very much at the beginning and after having played the game for more than 300 hours I can safely say I'm little bit sick of it. But if you like it that's great. I wish I could enjoy it as much.

 

[gudenau]

It's ADX, just drop in Sonic Adventure 2 music. :-D