What part of speech is muttering?

"Muttering" can be several parts of speech depending on how it is used in a sentence:

* Verb: This is the most common usage. Example: "He was muttering to himself."

* Noun: It can be used as a noun to refer to the act of muttering. Example: "The prisoner's muttering was barely audible."

* Adjective: While less common, it can act as an adjective describing something as being muttered. Example: "A muttering voice echoed through the hallway."

So, to know the exact part of speech, you need to look at the sentence it appears in and its function within that sentence.