Collocations for mumble - verb

incoherently
quietly, softly
quickly
sleepily
angrily, bitterly
sarcastically

about
I couldn't understand what he was mumbling about.

into
She mumbled something into her pillow.

to
George mumbled incoherently to himself.

Mumble is used with these nouns as the object:
apology, excuse, reply, word