0 'Mutter' is the normal one for 'under his breath'. 02br 00'Mutter' is usually complaining to yourself - if you've been told to do something but don't want to, but have to. 02br 00'Murmur' is quiet, similar to whisper, but not as conspicuous - for example if you wanted to make an ironic comment to a comapanion when you're meant to be quiet, you might murmur it.
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