Muffled as in either via the spell or the boot enchantment. I am wearing heavy armor and casting muffle, but I still feel like I am being noticed more than I should be. I was under the impression that, while heavier armor makes it harder to sneak, if you are muffled, you are just as silent no matter how heavy your armor is.

Here you go buddy:

http://www.uesp.net/wiki/Skyrim:Muffle_(effect)

Short version:

Yes, heavier armor is louder but as you get better at sneak and magic you should be harder to detect.

You may need quiet casting