For checking the transcript: https://www.buzzsprout.com/2379282
Under your breath
means to speak very quietly or mutter something so that others either can’t hear you clearly or aren’t supposed to hear you at all.

Example:
1- During a long meeting, her colleague whispered under his breath, This could’ve been an email.
2- When the teacher announced there would be a surprise test, Sam muttered under his breath, "Great, just what I needed.