Okay, so scratch what I said above in the comments. Apparently folks have figured out how Muffle works.
Muffle - 1.0
Muffled Movement - 0.5
Silence - 1.0
Nightingale Boots - 35.0 (level 32+ only)
Predator's Grace - 1.0
Everything else - 0.5
Apparently, any item enchanted by the player with the Muffle effect will have 1.0. This number is not affected by your enchanting level or gem size used on the enchanting process.
Noise from armor is then multiplied by
1 - sum of muffle effects, so if you have two items of 0.5 or one item of 1.0, your armor will make no noise. Any increase above 1.0, including via the best Nightingale Boots, has no additional effect. And yes, it will cut in half or completely eliminate noise from heavy armor just as well as light armor:
However, heavy armor is also noisier to move in than light armor, so it always adds a penalty to stealth. The Silence perk of the Sneak skill eliminates this drawback, as does the Muffle spell/enchantment.
Can you sneak well with heavy armor compared to light or is there no difference?
How does Silence work in the Sneak tree?
Do muffled garments guarantee complete silence?