Muttering is the indistinct grumbling sound of voices. The muttering in your math classroom might make it hard for you to concentrate on your work.
Muttering is usually unintelligible — you can't hear what people are saying, just that they're saying it. Muttering is usually also characterized by dissatisfaction. The continued muttering in an office may inspire a business's owner to offer every employee a raise, simply to put an end to the complaints. Muttering, like the verb mutter, comes from an imitative root, mut.