Thanks guys for your answers
It took me a long time to translate everything, but i'll answer to everyone 
A stroboscope would be cool.
: This can make basic light in addition to its main function of stroboscope ? If yes, it might be a good solution 
or even a piezo speaker can be used to pick up sounds and have an Arduino read the analog signal and control a string of LEDs. Google is your friend.
: OK thanks
I'll check on google. Piezo transducer = piezo speaker ? same thing ?
You may well be able to affix a piezo transducer direct to the back of the skin of the drum (if it is open bottomed)
: No it's not actually, but this will be
Some other guys advised me the pizeo transducer, I think I'll choose this
The question was not how to sense the beating on the drum nor how to trigger the light, it was which lights to use.
: it's ok, it also helped me to choose the sensor 
How bright do you want them to be? What will the ambient light levels be?
: I want them bright enough to project lights on the person who'll be playing on it. (the drum solo consists of 4 persons playing in front of the audience, with one 16x16 floor tom for each.) So I'll need 4 arduino ?
The ambient light will depend on where we're playing..
But we have our light engineer most of the time, so we can ask him to turn off every lights !
An LED strip works great for this stuff. Line it on the inside of the rim, hook up a 5V source (or 12V depending on which strip you use), with a small MCU that can read and translate the hits on the head and you have a product.
: hey man that's a good idea ! I didn't think about it ! I might choose this solution if there's no solution for my first idea of the project 
For a simple set-up, you'll need one input for each sensor and one output for each lamp. The basic Arduino Uno has 7 analog inputs and 14 digital input/output pins. That should be enough for your average drum kit.
: As I said earlier in this message, this is not for a drum kit
but for 4 floor tom. May be this will be more simple to have 1 arduino inside each tom ? For transportation, and to avoid a lot of cables on stage ?
- If you put the lights inside the drums, you'll see a nice effect, but I would guess your audience can't see the drum heads very well, except for the kick drum.
: I'd like to have this kind of brightness : http://youtu.be/bLUFg-kh4e0 - Time : 1:46 - Is this hard to do ? For the rest of your thougts, thank you, I'll think about everything you said and I'll reply to you later today 
Thanks !