MUTTER is a roleplaying verb. It can be used to mutter a single word as well. It can also be used with the preposition "about" and a single word.
>mutter You mutter something inaudible. Person mutters under her breath.
>mutter angrily You mutter angrily. Person mutters angrily.
>mutter about Halflings You mutter something about Halflings. Person mutters something about Halflings.
Note that mutter is also available as a [SPEECH] verb, making this possible:
>say =mutter Bah Person mutters, "Bah."
Which is preferable to:
>mutter Bah Person mutters bah.