Hi there " Miguelito ", and Welcome!
Congratulations on deciding to build your own place! Now for some reality...
The rooms are located on the second floor of a family home.
Building on the second floor is always a problem, and never as easy as building on the ground floor. Isolating a studio implies a very large amount of mass (weight), as Greg already pointed out. Reinforcing he existing structure to handle that weight is expensive. If you can't afford to do that, then you won't be able to get good isolation, and will have to settle for only "mediocre" isolation.
I'm sure the floor will last a lot.
Unless you are a qualified structural engineer, then "being sure" is not much use! You probably don't realize it yet, but you will be adding thousands of kilograms of extra weight to that floor. You DO need a structural engineer to examine the existing floor, and give you a written report stating what the current live load and dead load are, and what the maximum allowable live load and dead load are. That's the ONLY way you can be certain that about how much extra mass you can add. Guessing and hoping and "being sure" will not save your family downstairs, if that floor collapses one day from being overloaded, because you didn't get it checked...
There is Styrofoam + heating installation + concrete screed
So the existing floor up there is already a concrete slab? How thick? Is that structural? How is it supported? Are there beams of some type under the locations where you plan to have walls?
but I can install a mechanical ventilation system - I have experience in this.
Good! So the very first thing you need to then is to calculate the correct air flow rate and air flow velocity for each room of your studio, as well as the latent heat load and sensible heat load, so that you can define the correct AHU, register areas, and duct cross sections at the registers. Then you can use that to calculate the sizes of each of your silencer boxes, and the duct runs, then you can use that to calculate the static pressure, and check that the AHU can handle that static pressure. If not, you might need to change your choice of AHU to a high-static model, or reduce the static pressure of your system design, by changing duct sizes, duct length, registers, etc. Once you have that all worked out, then you can design the actual silencer boxes. The are very large, so it's important to know how big they will be BEFORE you start planning the wall and ceiling locations, as you need to leave space for the ducts and silencers. You say you have done this before, so you probably already know that, but for a studio it is even more important.
I do not need to insulate the room very much.
I think you didn't understand Greg's point: He was talking about ISOLATION, not INSULATION. Yes, you do need insulation as part of your isolation system, but it's not the same thing. It is possible to insulate a room without isolating it at all, and it is possible to isolate a room without using any insulation....
The very first thing you need to do is to define how much ISOLATION you need, in decibels. Everything else about your room depends on that.
But the wool in the walls will have a positive effect on the acoustics,
Actually, no it wont!

But even if it did, that is NOT what we are talking about here: we are talking about ISOLATING the room, to prevent sound from getting in and out. We are NOT yet talking about the acoustic response of the room itself. That's an entirely different thing.
in addition the rest of the floor in the house is not heated so wool will fulfill the function of thermal insulation.
That's irrelevant. Thermal insulation and acoustic insulation are two different things. Yes, the type of insulation used for acoustics does also have good thermal properties, but not the other way around. For example, the styrofoam in your floor has very good thermal properties, but ZERO acoustic properties: it is no use at all for acoustics, because it is closed-cell foam. Acoustic insulation MUST be open-cell (never closed cell), and must also have the correct gas flow resistivity properties (GFR), which has no meaning at all for thermal insulation.
It seems that you are confusing many issues at one here; Isolation is not insulation (two different things), isolation is not related to room acoustics (two different things), and thermal insulation is not necessarily any use for acoustic insulation (two different things).
The floor in the main room is 30cm concrete + 6cm styrofoam + heating installation + 5cm concrete.
So you already have a 2-leaf floor. Is the upper leaf full decoupled (acoustically) from the rest of the building? Did you calculate the MSM resonant frequency of that system, to check that it is low enough? Are you aware that if you add another floor on top of that with an air gap that you would be creating a 3-leaf system, which could potentially mae your isolation WORSE, not better?
...record many different instruments (guitars - mainly acoustic but also electric and bass, drums - djembe, congas, bongos,
Those are the loudest instruments, and out out a lot of sound power, and some of them also create issues with impact noise.
I started the construction of a wooden frame, under which I wanted to put the wool,
Are you talking about the walls? Or a new floor on top of the slab?
But I gave up because I'm afraid of the sound-drowning of the room and the lack of control over adaptation.
I'm not sure what you mean by "sound drowning". Please can you explain that.
I decided to use the box-in-box technique.
I want to use wooden beams and Rockton Acoustic wool
Why?
5cm in the entire main room, and 10cm in the vocal room. The whole will be covered with a plasterboards.
Why do you want a THINNER cavity in the LOUDEST room, and a DEEPER cavity in the QUIETEST room? I'm not following the concept here. Did you check the MSM resonant characteristics of those walls, to be sure that they will provide the amount if isolation that you need?
It's about the ceiling. I am an artist and a vision of doing something special appears in my head.
Studios are meat for sound. Yes, they should also look good, but the first priority in designing a studio should be to make it SOUND good. So first design it for good acsoutics response, then see what you can do with that design to make it look nice, aesthetically.
The main room has the ability to do a high ceiling of about 15ft (at the top). However, the roof above room is irregular. Or I can do a regular ceiling, using plasterboard and Rockwool Rocton wool.
The control room ceiling can be irregular if you want, and it can be sloped if you want. However, it MUST be symmetrical, and if you do slope it, then it needs to be lowest over the speakers and console, sloping UP to be highest at the back of the room.
1. Main question: What is your opinion about the ceiling? Should I make a regular flat ceiling at 260cm, do I use the opportunity I give to raise the ceiling?
As Greg already said: if you can raise it, then raise it! However, those roof trusses will need to be modified to allow you to raise the ceiling, and that is going to be expensive. Once again, you will need a structural engineer to tell you how to do those modifications. Some of my customers have modified their roof trusses for studios that I designed for them, so it can be done, but it is not simple, and not cheap. Do NOT change anything on those roof trusses without first consulting a professional structural engineer. Roof truss carry very large loads, tensions, and stresses, in all directions at once. Messing with a roof truss can cause not just the roof to collapse, but also the walls.
I want to leave gaps from the floor and ceiling so that the walls can work.
I'm not sure what you mean by that. Your inner-leaf walls cannot have any gaps. Not even tiny ones. They must be fully sealed, hermetic. Completely air-tight.
- Stuart -