I just needed to add a couple of lines of code.
tinymce_frame = driver.find_element(:id => "Speakers_ifr")
driver.switch_to.default_content # Added this line
driver.switch_to.frame(tinymce_frame)
editor_body = driver.find_element(:tag_name => 'body')
driver.execute_script("arguments[0].innerHTML = 'bob'", editor_body) #Added this line
My next challenge is trying to make 'bob' a variable that is being passed into the function. I'll write that up as another question, if I cannot find it.