I have for you amazing sound of a summer rain storm.
How to install rain.wav to your game.
In the folder where the game is installed you have to find the folder “sonds” (For Windows7 C: \ Program Files (x86) \ Farming Simulator 2015 \ data \ maps \ sounds). In there you need to replace the files rain.wav. and that is all.
Credits: seba j
rain (file size = 56 MB)