spec. With closed lips, inarticulately.

With confused and indistinct articulation; also, with a husky or hoarse voice.

In a mumbling manner; indistinctly.

in a grumbling manner; †mumblingly (obsolete).

With a mutter; in a muttering voice.

in a clipping manner.

In predicative use: muttering.

In a spluttering manner.

In a splodgy manner. Also figurative.