Have found a script play the rain sound: llLoopSound("54a0a199-0a1b-4b23-aa82-9d6a5ddc29c1", 1.0);
Make the sound in the prim content and in the script UUID of the sound and the volume (0.0 = silent 1.0 = loud)
llLoopSound Plays attached sound looping indefinitely at volume
oh. and in Texture Trees and Scripts? (friends-grid, i think) one day, i‘ve found there a frog, that did make rain with thunder, when you touched it, perhaps you can pull it in parts?)