"Under one's breath" is a phrase that implies talking softly, often in a way that indicates frustration or anger. There are a few different ways to express this concept. For example, you might say someone is mumbling, muttering, grumbling, or whispering. If they're being particularly incisive or sarcastic, they might be sniping or snarking. On the other hand, if they're expressing something more deep-seated, such as regret or remorse, they might be murmuring, sighing, or even whispering a prayer. No matter what words you choose, the key is to convey that the speaker is saying something that isn't meant for public consumption.
