1. First follow the instructions to install RetroPie BGM Player v2.0 @ https://github.com/Naprosnia/RetroPie_BGM_Player

2. The installation will be successful however it'll complain about autostart.sh, sadly the x86 RetroPie version
doesn't actually use or have an autostart.sh which is where the fun begins.

3. What we're gonna do is hijack the .desktop auto-start symlink and tell it to run EmulationStation and play music.
First create an autostart.sh file in your /HOME/.config/autostart and paste the following inside of it:

#!/bin/bash
nohup $HOME/RetroPie-BGM-Player/bgm_system.sh -i --autostart >/dev/null 2>&1 &
pid=$!
exec gnome-terminal --full-screen --hide-menubar -e emulationstation

4. Now you need to enter the terminal and go to your /HOME/.config/autostart/ folder and type:

sudo nano retropie.desktop

and replace the Exec= line with this:

Exec=/home/<your user>/.config/autostart/autostart.sh

Ctrl-O to save and Ctrl-X to Exit

MAKE SURE <YOUR USER> IS CHANGED TO YOUR LOGIN/USER NAME.

Now RetroPie will play music on start, however we aren't done yet!

5. The next task is to replace runcommand-onstart.sh and runcommand-onend.sh
so proceed using terminal to your /opt/retropie/configs/all/ folder and type

sudo nano runcommand-onstart.sh

and replace everything inside with this:

bash $HOME/RetroPie-BGM-Player/bgm_system.sh -s
pkill -9 audacious

Ctrl-O to save and Ctrl-X to Exit

Now type:

sudo nano runcommand-onend.sh

and replace everything inside with this:

bash $HOME/RetroPie-BGM-Player/bgm_system.sh -p
nohup $HOME/RetroPie-BGM-Player/bgm_system.sh -i --autostart >/dev/null 2>&1 &

We now have the the music script shutting down on launching an emulator and restarting itself when coming back to
EmulationStation.

6. The only thing left to do is have EmulationStation kill the music on exit, so we will now bring our terminal to the
/opt/retropie/supplementary/emulationstation/ folder and type

sudo nano emulationstation.sh

Before the exit $ret line insert

pkill -9 audacious

Save & Exit.

That's it! Enjoy.
All credit goes to andrew from the Redream Discord.
    - RennyC