Midpoint of a segment
Your definitions of make-segment and make-point can be simply bound to cons; you may do the same for accessors.

(define make-segment cons)
(define start-segment car)
(define end-segment cdr)

(define make-point cons)
(define x-point car)
(define y-point cdr)


Your definition of sum is similar to how + works.  You may use + instead in your definition of average.

(define (average . l) (/ (apply + l) (length l)))


Notice the repetition of code in your definition of midpoint--you create a new point out of the averages of each coordinate of the two end-points that describe the line segment.  You may abstract out the calculation of averages either by writing your own helper function or by using the built-in map function, like so:

(define point-&gt;coordinate-accessors (list x-point y-point))
(define (midpoint seg)
  (apply make-point (map (lambda (point-&gt;coordinate)
                           (average (point-&gt;coordinate (start-segment seg))
                                    (point-&gt;coordinate (end-segment seg))))
                         point-&gt;coordinate-accessors)))


This definition is general enough to handle midpoint calculation in as many dimensions as handled by make-point and point-&gt;coordinate-accessors.