Do muffled garments guarantee complete silence?

Yes, it is possible to learn the muffle enchantment. Note that you can only put it on footwear though.

The reason why you can't get it off of the dark brotherhood armor is because you are not allowed to disenchant it. If you find a pair of random boots with the muffle enchantment on it, you can disenchant it to learn the effect.

Your enchantment has a hidden magnitude effect that lowers your armor weight penalty by a % (from 0-1). A magnitude of 1 will mean you suffer no noise penalty to armor at all.

Note, you can reproduce this effect with the muffle illusion spell and/or the silence perk.

Yes, muffled garments do guarantee complete silence. By the way, the only thing you can muffle is boots — only those you find though, not DB boots or TG boots.