Muttering is not typically considered onomatopoeia. Onomatopoeia refers to words that phonetically imitate or resemble the sound they describe, such as "buzz" or "hiss." While muttering suggests a low, indistinct sound, it doesn't mimic a specific noise in the same way that onomatopoeic words do. Instead, it describes a manner of speaking rather than a sound itself.
The word whoosh is considered and classified as an onamatopoeia. An onamatopoeia is a word that is written out to describe the way that something sounds.
I could not understand what it was that you were muttering about.
Onamatopoeia
Bang
Muttering to oneself is a complex vocal tic
1+1=2
onamatopoeia
After the fight with her boyfriend, Olivia was left muttering arguments and comments under her breath.
speaking indistinctly and complainingly
3
An example of a sentence for an onamatopoeia is : "The pipes went clunk and clinck at the old house across the road."
Uttering, muttering, fluttering, cluttering, and sputtering.