sound off vs speak up

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. 

speak up

verb
  • To make oneself or one's opinions known; to advocate or assert oneself. 

  • To talk more loudly or plainly. 

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