Alright well I don't think you can actually add sound to the weather through Sky Maker.
What I can suggest is going to Object Maker >> Effect Maker >> Particle and type in
Weather/Rain. This will load the rain as a particle, just adjust it to the wanted size and height, I would recommend making the X and Z coordinates 1000 or larger. It doesn't matter how high you make it because the rain just keeps falling until it touches the ground.
From there you can add the sound to it. However, because this is just the loaded particle there are no weather cycles so it will rain nonstop in your map. A good thing about is that you can choose where in particular you want it to rain in your map.
Hope you find this helpful