mutter under one's breath 低声嘀咕
mutter to oneself 自言自语
A: Did you hear what he said?
A：你听到他说了什么吗？
B: No, he only mutters so softly.
B：没有，他说话声音太小了，只是在嘀咕。
1) 低声嘟囔
2) 喃喃自语
3) 不满地抱怨
1. 嘀咕，喃喃细语
verb
1. To say something in a low, quiet voice, especially when you are unhappy or annoyed.
Synonym: mumble, murmur