Right click on Earn to Die 2, then go to Manage, Browse local files.

Once you found it double click it, normally it's at the top.

There will be 6 music files, but you can't add more than that. If you want, you can try to listen to it.

MudFace.mp3 is the music that you hear in the garage.
The others are played while you're driving.

Next, it's time to add your own music.
The music file MUST NOT be longer than 5 minutes.

After you add it, you MUST rename it to any music files that you want to replace. Mine is MudFace.
Add "(ORIGINAL)" to the end of the original music file, before you replace them. So you can go back and use the original music at anytime.

Lastly, enjoy your music.

Thank you very much for reading this guide that I made, it's the first guide,
so if you have any feedback, please send it down in the comment.