Every time I go camping and it starts to rain, I can't sleep anymore because the raindrops that fall on the tent causes a really loud and annoying sound. Is there any technique or tool to avoid or at least decrease that sound?
To me it's one of those sounds that can be soothing initially, but start to grate on you after hearing it for hours. You could try ear plugs. I don't wear them because I like being able to hear my surroundings, but I often see people camping with mp3 players, so not everyone appears to feel the need to be aware of their surroundings!
I usually throw a tarp above my tent. (an additional tarp, not the one that comes with most modern tents). I find that it usually muffles the sound of the rain. You will definitely still hear but not as intense. They are relatively cheap too ($15+ CDN).
Sometimes just choosing the location of your tent can make a huge difference in the amount of rain it receives. For example, you can protect it by placing it under thick foliage, or near a cliff if the wind is in the right direction.
I would throw some type of foliage over the tent, to "break" the fall of the rain... so long as your tent is strong enough to support the weight!
Sometimes if you just can't stand the sound of nature, an mp3 player or other source of noise that you CAN stand may work.