(UPDATED) change the main menu music

Alright folks, now you will learn how to change the main menu music to any WAV or MP3 music you like.

How does the game know which file to play?

The main menu music file will be defined by what is written inside a XML file called "AudioDefines". Its name is pretty self-explanatory - this file defines what audio file (filename) will be used to what in-game sound entity.
However, there are up to three AudioDefines files in total. One for the vanilla game and one for each expansion you are using.
For most of us who have GnK and BNW, the correct file will be located here in the BNW expansion folders:

(...)\Steam\steamapps\common\Sid Meier's Civilization V\Assets\DLC\Expansion2\Sounds\XML

Just like there is an "Expansion2" folder, there is an "Expansion" folder for GnK only. And for the base game, the relevant file is here:
(...)\Steam\steamapps\common\Sid Meier's Civilization V\Assets\Sounds\XML

You will see that this XML file tells the game to load a file with a specific name, and it's going to look for it inside the "Sound" folder.
In our case, we can see the game loads a file called "OpeningMenu_Exp2".

Ok. But what exactly do I do?

UPDATE! It is not necessary to edit the XML files like I previously suggested. It still is, however, an option, to edit the name of the file the game looks for in the part below of the file:

<SoundData>
<SoundID>SND_MUSIC_MAIN_MENU_EXP2</SoundID>
<Filename>OpeningMenu_Exp2</Filename>
<LoadType>STREAMED</LoadType>
</SoundData>

So, the actual file of the menu music will be found here:

(YourDrive:)\Steam\steamapps\common\Sid Meier's Civilization V\Assets\DLC\Expansion2\Sounds\Streamed\Music

It's called OpeningMenu_Exp2.wav.
Just rename it to something diferrent (add a "_old" suffix to the file will do), and copy a .wav or .mp3 into the same folder, and rename it to the name above!
The game will now load the file with the same name as defined it its XML files, no matter it it's a WAV or MP3 file. I have tested it with a 50Mb 45 min long MP3 and it worked fine.
Note: I tested it with an AAC music file and it did NOT work.

Simple. The original file was a music mix I got from the internet, but now Civ V loads it in the menu.

FAQ

• --- I want to use the vanilla game menu music, where is it located?

(YourDrive:)\Steam\steamapps\common\Sid Meier's Civilization V\Assets\Sounds\Streamed\Music
It's the file called OpeningMenu_Proto_1.wav
Just replace in the appropriate folder and rename.

• --- I don't have any expansions, what file do I need to replace?

The file above

• --- I would like to use a leader music / war / peace music of the game as my main menu music.

Since the MP3 files are scattered inside the main game directory (across DLC folders, expansion folders and so on), I suggest doing an advanced search for all large Mp3 files.
Just search for "*.mp3" at the main Civ V folder (include the asterisk), then on the top bar of the explorer window you will see the "Search" tab. Click "Size" and select "large, above 1MB" and go. Now you should only be seeing music MP3 files.
Note: text will vary according to your Windows language.

• --- Can this corrupt the game files? Is this considered cheating? Will it affect my previous saves?

This is a simple tweak replacing file, it does not alter the gameplay / saves in any way. I consider this safe, but I do recommend backing up.

• --- Will the game eventually revert these changes back?

Yes, if you have to verify game integrity, it will download the original WAV files and replace them. It also re-downloads the XML files if you have altered any.

[Ctrekoz]

Haha, I just thought I've already made a comment in the past, and wondered if it was too hateful towards BNW theme and you've deleted it :P. Thank you again! I've even tried to make 1 hour 700 mb compilation of various themes and it works too!

[Matta Leão]

Hi @Ctrekoz, I didn't delete any comments here. Glad to know these steps still work! Cheers

[Ctrekoz]

Have you deleted my comment? Anyway, BNW menu is so boring to me, makes me really annoyed, so thank you very much. ATM I've set Civ V Alexander Peace Theme.

[the Reggienator]

a game better than its sequel

[your local ariral]

oh nvm i thought this was among us lmao

[your local ariral]

um... what is Sid Meier's Civilization V

[AtomicSpeedFT]

Thank you so much!

[Matta Leão]

@AtomicSpeedFT Updated just for you. Thanks to @VieRuz for reminding us of this method.

[AtomicSpeedFT]

Custom music ever coming?

[Anti-Anti-Christ Gaming]

@VieRuz The Real MVP