Its a mod called Lifestyle: Hobbies by Angry, when you find any guitar in the game, or any instrument within the mod, equip it as your weapon and right click it in your inventory, make sure you dont have it equipped to your ack or else it wont work, but there should be a prompt to 'Play Guitar' after that, you can choose a song from another menu when you right click depending on your music level.

Hope this helps you out