Each backer gets the chance to submit songs to the game. To submit songs, simply comment with (up to) 3 songs you'd like to see in the game in the comments section of the Kickstarter campaign.
Every few days or so we'll compile everyone's suggestions into a survey and send it out the backers. Backers will select songs on the survey they want to see in the game. We'll see what songs get the most votes and put them into the game.
Don't see the answer to your question? Ask the project creator directly.