General Synonyms:
* Murmur: A soft, low, indistinct sound.
* Grumble: To complain in a low, unhappy voice.
* Stammer: To speak with difficulty, hesitating and repeating sounds.
* Stutter: Similar to stammer, but with a more noticeable repetition of sounds.
* Slur: To speak in an unclear, indistinct way, often due to intoxication.
* Whisper: To speak very softly.
* Mutters: To speak in a low voice, often to oneself.
* Rumble: A low, continuous sound.
Informal Synonyms:
* Gutter: To speak in a way that is hard to understand.
* Bumble: To speak in a confused or unclear way.
* Spit out: To speak words quickly and unclearly.
* Ramble: To speak in a long, rambling way, without much focus.
More Specific Synonyms:
* Articulate: To express oneself clearly. (This is the opposite of mumble.)
* Elucidate: To make something clear or understandable. (Opposite of mumble)
* Speak plainly: To speak directly and honestly. (Opposite of mumble)
* Declare: To state something formally or officially. (Opposite of mumble)
* Announce: To make something known publicly. (Opposite of mumble)
The best synonym will depend on the specific situation and the intended meaning.