Riddle me this:
I am a word with the following properties.
Find me and solving riddles might be one of your qualities.
An anagram is what you must be to execute me.
An anagram is a form of art if you then add the letter C.
An anagram is a number of items if you rid the last two.
An anagram is a sound if you then add E, H and W.
An anagram does not move if you add the letter L.
But, do not forget, you must then discard E and N as well.
I have six envelopes, but you cannot write in them.
Is it too difficult to try and guess what word I am?
I made up this riddle for fun, as well as the following poem.
My riddles have not been as hard as I thought they would be.
So I made this riddle a lot harder, well, hopefully.
(In my riddles, I also love to incorporate a bit of poetry.)
Hope you enjoy!
Hints for the answer are below.
I do not require much, just only the sides;
But if you are not fully aware, then trouble may decide.
I added the word
then to lines $3$ and $6$ in order to avoid confusion of when to add letters.