mumble
n.a low, unclear sound that is hard to understand. You use this when someone is speaking too quietly or too fast to be heard clearly.
n.a low, indistinct, or unintelligible sound, typically resulting from speech that is too quiet or too fast.
“I could hear his mumble from across the room.”
“The professor's mumble about the exam schedule was barely audible over the noise of the fans.”
“The witness's mumble was the only evidence the jury had regarding the suspect's presence at the scene, though it was difficult to determine if he was speaking to himself or to a companion.”
From Middle English momelen, a frequentative of mum (sense 3) (“silent”). Compare German mümmeln, Middle Dutch mommelen and Dutch mompelen. Perhaps related to Norwegian Nynorsk mumpa (“to gum, to chew without teeth”).
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.