NOTE: You can only add custom music only if the mod has a folder for it.

Find your mod folder (Basically, it's Project D 2.3).
Go to Program Data/rFactor/Music/Project_D 2.0
There, you can see 3 Music Files. It's either you replace them with new ones, or put new music without deleting them.

And there you have it! A custom song for rfactor!

You can also delete songs and replace them with new ones, or simply add new ones.
Refer to the compile documents under the Utilities Section, you can get there the Menu Music from S2U (Shift 2 Unleashed). The Music suits the game itself...trust me :)And as a final note, those are only menu BGM's. You can't play music while you're in-game.

Thanks to John Kai Tolentino for the tutorial!

Any other questions, please feel free to open a topic!