I edited this in the browser so I might not be quite right, but I think I got it.

bob.rb

class << Silent = Object.new
  def heard?(other)
    other.empty?
  end

  def respond
    "Fine. Be that way."
  end
end

class << Question = Object.new
  def heard?(other)
    other.end_with?("?")
  end

  def respond
    "Sure."
  end
end

class << Shout = Object.new
  def heard?(other)
    other == other.upcase
  end

  def respond
    "Woah, chill out!"
  end
end

class << Default = Object.new
  def respond
    "Whatever."
  end
end
  
class Bob
  def self.add_thought(thought)
    @thoughts.push thought
  end
  @thoughts = []
  
  add_thought Silent
  add_thought Question
  add_thought Shout
  
  def hey(phrase)
    think_about(heard(phrase)).respond
  end

  def heard(phrase)
    String(phrase)
  end

  def think_about(might_have_heard)
    thoughts.find(-> { Default }) { |thought| thought.heard?(might_have_heard) }
  end
  
  def self.thoughts
    @thoughts
  end
  
  private
  def thoughts
    self.class.thoughts
  end
end

@JEG2, thanks. I think that is a good idea. Something like:

  @responders = []
  def self.add_responder(responder)
    @responders.push responder
  end

  add_responder Silent
  add_responder Question

I think that could be good. I also like the idea of adding multiple responders. Maybe this would be better to do on an instance?

@JEG2 is this new version what you were thinking?

Oh, I read that all wrong. You said to define responses.

  respond_with "Fine. Be that way.", when: ->(other) { other.empty? }