I suppose it is all the objects a person's feet interact with that make the noise, not the person herself. But if that is true, what would a spell for silent walking be? T: Room isn't right either because everyone would be silent...
It is the perfect spell for sneaking, as it prevents you producing any sounds. It also gives a massive penalty to spellcasting and stops you speaking until it wears off, so it's great to cast on your grogs but as a magus you'll probably want quiet casting or a handy spell of "unravelling the form of imaginem" mastered for silent casting before you go around with veil of invisibility and silence of the smothered sound stacked.
I like doing this as T: Part. You don't make the whole being incapable of making sound, just everything except the head. Yes, that adds a magnitude, but for a sneaky magus R: Personal drops it by 2 so it's down to PeIm 15. If you can maintain spells well, dropping it to D: Concentration lowers it to PeIm 10.
You are incapable of producing sounds(and supposedly your clothing, or armor) however any foilage creaking or snaping back, stones rattling away etc is not covered. Are you producing the sounds or is it more often your interaction with the surrounding enviroment that causes the sound? Or so I've read it.
As a basic idea, I would think so. D: Momentary should generally do since it will handle several seconds. You might need/want T: Part, though.
Also, I wouldn't list it as +2 unlimited because going that route you'll have to do things like +0.3 sometimes for uses. On top of that, if the level is below 5, the magnitude shift won't work properly at all. The books typically switch from commas to semicolons after the spell effect. What follows the semicolon are level increases for the instilled effect. So what you wrote would more typically be (Base 3, +1 touch; +10 unlimited uses) for a level-14 instilled effect.
The silent boots seems good for silencing the ground or any twigs you may step upon. However it still leaves you brushing against branches, creaking, leaves rustling. Which honestly feels like they are what makes the most sound, if you are sneaking, positioning, knowing not to get close to bush is probably as big a part as 'putting down your fot lightly'. Boots of Silence should be fine for sneakily galloping down the streets of Rome without being heard and similar situations where there are nothing to brush against and thus i wonder;
Can we get a 'sphere of silence' going somehow? Silencing everything within a pace of you? I seem to recall some discussion about similar things this using size modifiers and/or part to achive an 'aura' like effect hermeticly, don't recall if there was any mysteries involved.
You'd need to create a moving volume of silence including the feet and parts of any objects that they touch,
Perdo imaginem uses a +1 modifier for a moving target for invisibility spells. The Perdo Herbam Spell Treading the Ashen Path has a similar target and it's spell design has a +1 magnitude modifier for:"fancy effect (the spell effectively keeps being cast while the caster walks seven miles)"
I think that this wouldn't raise too many eyebrows
Adding in the range makes it level 25. Thinking about game balance I'd be tempted to change the target to special equivalent to part +1 to lower it back down to 20. It doesn't feel like it should be harder to pull off than veil of invisibility.