Mumble (verb)

1. Speak indistinctly or with a low voice, making it difficult for others to hear or understand.
2. Speak in a low indistinct voice.

Origin:

Late middle english: imitative.

Examples:

1. He mumbled his response, so I couldn't hear him.
2. She mumbled something under her breath.
3. He mumbled an apology.
4. I couldn't understand what he was saying because he mumbled.
5. He mumbled through the entire meeting.