General synonyms:
* Murmur: A soft, low, indistinct sound, often difficult to understand.
* Grumble: A low, complaining sound, often expressing dissatisfaction.
* Stumble: To speak in a hesitant or confused way, often tripping over words.
* Slur: To speak in an unclear or indistinct way, often due to drunkenness or a speech impediment.
* mutter: To speak in a low, indistinct voice, often to oneself.
* Whisper: To speak in a very soft voice, often to avoid being overheard.
More specific synonyms:
* Stammer: To speak with difficulty, often repeating sounds or words.
* Sputter: To speak rapidly and incoherently, often from anger or confusion.
* Gibber: To speak in a confused or nonsensical way.
* Bumble: To speak awkwardly or clumsily.
* Gutter: To speak in a low, harsh, or unpleasant voice.
* Drone: To speak in a monotonous or boring voice.
The best synonym to use depends on the specific context and the intended meaning. For example, if someone is speaking very softly and it's difficult to understand what they're saying, "murmur" might be the best choice. If they are speaking in a complaining tone, "grumble" might be more appropriate.