mouth off vs sound off

mouth off

verb
  • To complain or otherwise express oneself in a loud, immoderate manner. 

  • To talk impudently, especially to one's superior. 

sound off

verb
  • To hold forth about something in an opinionated manner. 

  • Command instructing a person to confirm that they are present or that some other objective has been met. 

  • Command instructing a person to speak up or acknowledge something. 

How often have the words mouth off and sound off occurred in a corpus of books? (source: Google Ngram Viewer )