Talked under one's breath
Spoke in low unclear voice
Spoke in low unclear tone
Spoke in low unclear voice, uttered subdued grumble
What a low way to have been carrying on in manner of speaking!
But Lo! By the sound of it, thus spake a thousand
To have said anything so low would have turned your stomach and tree'd you
It's grand to have said anything so low as this
How grand to have said this in such a low way!