Unpack ADXEncrypter into a folder. For best effect, copy your prepared ADX file/s into the same folder.
Important! You must have the latest Java Runtime Environment installed to execute ADXEncrypter.
Once you have everything set, simply drag-and-drop your ADX into the file named 'GvsGNextPlus_PresetA'.
Something like the following should appear. This confirms that the process occurred without error.
Once all your ADX files have been encrypted, they can be compiled into the game.
Before that, a naming convention must be considered. The game uses a hexadecimal numbering system for the BGMs, with audio for the series going 10-19, then 1a-1e...going something like:
bgm10 - MS Gundam
bgm11 - Zeta
bgm12 - ZZ
etc...in order of their display on the BGM selection menu (rightward).
For the 'Original' BGMs:
bgm1f - Original 1
bgm3b - Original 2
bgm3c - Original 3
bgm4f - Original 4
bgm50 - Original 5
bgm51 - Original 6
bgm52 - Original 7
bgm53 - Original 8
bgm54 - Original 9
Rename your ADX files to whatever place they will be replacing.
Once done, you can now begin compilation. For compilation, we will be using UMDGen and DKZ Studio.
If you haven't done so already, rip your game from the UMD, and open up the container using UMDGen.
Extract 'X_DATA.BIN' somewhere by right-clicking on it, then selecting 'Extract'. Go to where you extracted it, then rename it as 'X_DATA.AFS'.
NOTE: You may have to change Windows' option of displaying the extensions of known file types. If you see the extracted file as 'X_DATA.BIN', then you can just change 'BIN' to 'AFS'. But if you only see 'X_DATA', then you'll have to switch the Windows option. Search Google for more info on doing that.
Once X_DATA.AFS is ready, open the file in DKZ Studio.
Simply drag-and-drop all your ADX files, saying 'Yes' (or 'Yes to all') to any replace prompts.
Save the file, then rename your saved file back as 'X_DATA.BIN'. Drag-and-drop it back into your GvGN+ Disc Image in UMDGen - in the same place - overwriting the old copy. Save the new Image, then copy it into your Memory Stick.