Here are some possibilities depending on what you mean:
If you're talking about the sound:
* Silence: This is the most straightforward opposite.
* Whispering: A very quiet sound.
* Humming: A low, steady sound.
If you're talking about a texture:
* Smooth: The opposite of a rough, crackling texture.
* Soft: A delicate, non-crackling texture.
* Silky: A very smooth and flowing texture.
If you're talking about a visual effect:
* Solid: The opposite of something that is cracking or breaking apart.
* Unbroken: This refers to something that is whole and not cracked.
* Flawless: This describes something that is perfect and without any cracks.
Can you provide me with more context about how you are using the word "crackling"? That will help me give you a more precise antonym.