I find very interesting the possibility to destroy the ability to an object to emit sounds using Perdo Imaginem, but I do not succeed to understand as he works, so if I would like to silenciate my boots or the hinges of one door wich spell I have to use for a small duration of two minutes (diameter duration)?
If you would like to silence your boots or the hinges of a door, the spell that you'd use would be a PeIm, with a Target equal to whatever you want to be silenced.
PeIm Guidelines
Level 3: Destory an object's ability to affect... hearing.
+1 Magnitude for "changing" images (of any sense)
So, for instance...
Door, only long enough to open it:
Base 3, +1 Touch, +1 Concentration, +0 Individual*, +1 changing = PeIm 10
Boots
Base 3, +1 Touch, +1 Diameter, +1 Part*, +1 changing = PeIm 15
- Target- by definition (last par. on p 143), "A base individual for Imagonem is the equivalent of an adult human being..." I'd treat the boots as "part" (rather than Individual) because they are less than the whole of a human- the noise their feet make is only a "part" of the noise they normally make. This allows them to speak and make other noises.
And the boots are not "Group", imo, because it's a) based on the noise 1 human makes, and this is certainly does not exceed that, and b) it's is far, far less than the noise that 10 humans would make (a "group" worth of individual for Imagonem).
To silence two separate boots on 2 separate individuals would require Group. Same number of boots, but a different situation in the eyes of "magic".
Note that to "deafen" someone would normally be either a PeCo or PeMe (I, personally, could see either one working- one destroying the ear's ability to hear, the other the minds ability to understand sound). But also, possibly (probably?) some sort of PeIm "ward" around them to destroy the sound. I'd think you could also define an Ignem ward to wall noises in, so that, say, a group could speak in private, even shout or fight, without the noise escaping the area defined by the ward. (Cone of Silence, anyone?)
Compared with the effect that the spell creates the magnitudine is amazingly low! In game using spells like these could help very much the caster or his grogs and practically every low level wizard is able to cast them with little effort.
Try to think to a rogue owning a pair of enhanced boots, he could ambush a lot of persons from their back hitting them with a dagger in the middle of their shoulders... It is not umbalanced?
how can a Storyteller counter his players from using these spells or make the thing a little bit more difficult?
One more question, I have not understood where you have taken the "+1 changing" modifier and in which conditions it is applied.
Read the PeIm guidelines, 2nd paragraph. If an "image" (that affects any sense) is constant, there is no modification, but if the image is "changing", then add +1 magnitude.
Since boots don't make noise constantly, nor door hinges squeek constantly, those images are "changing".
How to make it "more difficult"? This question is similar to the recent "sleep = instant kill" thread. Magic is powerful. Even small effects can make a huge difference for the clever mage.
As a SG, I would consider the following:
- Only the boots are silenced. Not the clothing, not the breathing. While these make very little noise, they are not perfectly silent, and so there is some small chance that the intended victim may react.
And if we consider an entire assassin silenced, not just his boots, then:
-
A silent assassin does not make a silent victim. A shout of pain or alarm, or the sound of a falling body (or armour?!) can draw additional attention, even if the victim dies from the attack.
-
A well-guarded location will have more than one guard. If guard #2 sees guard #1 drop, not even invisibility will prevent the alarm.
-
The Code. If rumour escapes that some lord had guards mysteriously assassinated, the Quaesitores might(?) get word- if they do... "I will not endanger the Order through my actions"- which could happen if magi get a reputation for assassination. "
"Nor will I interfere with mundanes..."- if the local lord suspects, even without proof, that the magi are responsible, it's "bad". If the Quaesitores arrive, the magi are in big trouble.
Magi are powerful. If they want to kill someone, they usually can, no question. In many different ways. Personally, or via servants.
What happens next is larger than "and the body falls to the ground".
Good gaming. 
Experience shows that there is little need to. It may be fun to break into a castle unseen, unheard and unsmelled. But it gets pretty boring after that.
Style WILL get the upper hand here in time. You'll have magi using Imaginem to camouflage instead of getting invisible, magi to use Ignem to hide in the shadows and so on...
If your players overdo it with invisibility and being unhearable, there is a dozen of completely mundane ways to get them caught anyway.
How to unsilence the magically silent:
Sure, his feet do not make any noise. But if he stumbles onto that string, the half-dozen bottles/pans/bellows/whatever attached to it WILL make noise.
The dog may not hear em, but he'll smell em. The following bite ensures enough noise, as does the dogs barking.
How about caltrops?
How to see the unseen:
Throw some flower in the air. See their footsteps on the ground.
If your rich, throw pepper and hear them sneeze, conveniently blinding them in the process...
You get the idea...