Charlie_Guitar.rb

class Charlie
  def initialize(guitar)
    @guitar = guitar
  end

  def play(guitar)
    guitar.type == 'electric' ? guitar.sound.upcase : guitar.sound.downcase
  end

end

class Guitar
  attr_accessor :type

  def initialize(manufacturer, color, type="electric")
    @manufacturer, @color, @type = manufacturer, color, type
  end

  def sound
   'twang'
  end
end

g = Guitar.new('Gibson', 'blue')
c  = Charlie.new(g)
p c.play(g)

#Method sound would make more sense to be in class Guitar because it's a guitar action.
#Added guitar to sound.upcase and sound.downcase so it could find the method in class Guitar.
#Attr_accessor :guitar is unnecessary because the guitar attributes were set properly with def initialize(guitar) 
#Attr_accessor :manufacturer, :color are unnecssary because the attributes are not called from within class Charlie. The type attribute is called from class Charlie so it is needed.

1. yes
2. The Guitar class should provide everything for a guitar object. I like to say that "an object should know everything about itself". So -- the logic for sound should all be in the Guitar class
3. yes, but a lot of folks would make an accessor for :guitar, which I think is ok, except it would only have to be a reader. One very common mistake I see is that engineers make attr_accessors where they need only attr_readers or attr_writers
4. sort of. As you'll see, we won't use any accessors from the Guitar class in the corrected solution here:

class Charlie
  attr_reader :guitar

  def initialize(guitar)
    @guitar = guitar
  end

  def play
    guitar.sound
  end
end

class Guitar
  def initialize(manufacturer, color, type="electric")
    @manufacturer, @color, @type = manufacturer, color, type
  end

  def sound
    s = 'twang'
    @type == 'electric' ? s.upcase : s.downcase
  end
end

But again, I feel that it is probably better to do as you suggest, and simply use the @guitar instance variable inside the Charlie class, as there is no need to access a guitar attribute from outside that class.