facing the same problem, i lately post my short solution:

(define (map-nested f L)

    (define (map-nested-aux x)
      (if (list? x)
           (map map-nested-aux x)
           (apply f (list x))))

    (map map-nested-aux L))

example:

(map-nested sin '(0.1 0.2 (0.3) 0.4))
'(0.09983341664682815 0.19866933079506122 (0.29552020666133955) 0.3894183423086505)