Go here for all your needs: https://gbatemp.net/threads/loadiine-gx2.413823/
Loadiine GX2 lets you replace the internal resources to theme the loader and replace the pictures or sound.
Just put the files you want to replace in the resources folder and it will be loaded in memory instead of the included resources.
A list of resources filename can be found on github data folder.
Note about the background sound:
If you place the BG sound into the resources folder, it will be loaded in memory at launch.
You can tell loadiine to stream a file from SD card instead of loading it into memory by editing the BgMusicPath option in loadiine_gx2.cfg file.
The compatible format are mp3, ogg and wav.
Example : BgMusicPath=SD:/sounds/my_own_bg_sound.mp3