muttered
Definitions
To speak in a low, barely audible voice, often in a discontented or irritated manner.
To utter words indistinctly by barely moving the lips.
Examples
He muttered an apology under his breath and walked away.
The old man muttered to himself as he searched for his misplaced keys.
Synonyms
mumbled whispered murmured grumbled muttered under one's breath