You’re correct that s-reverse does not exist in Audacity’s version of Nyquist.
For short sounds you can convert the sound to an array, reverse the array, then convert the array back to a sound. Example:

(defun s-reverse (sig)
  (setf *MAX-REVERSE-SAMPLES* 1000000)
  (let ((ln (snd-length sig (1+ *MAX-REVERSE-SAMPLES*)))
        (srate (snd-srate sig)))
    (when (> ln *MAX-REVERSE-SAMPLES*)
      (error "*MAX-REVERSE-SAMPLES* exceeded"))
    (let ((ar (snd-fetch-array sig ln ln)))
      (macrolet ((swap (a b)
                   `(let ((tmp ,b))
                      (setf ,b ,a)
                      (setf ,a tmp))))
        (do ((i 0 (1+ i))
             (j (1- ln) (1- j)))
            ((>= i j) (snd-from-array 0 srate ar))
          (swap (aref ar i) (aref ar j)))))))

;; Apply the function to the selected audio:
(multichan-expand #'s-reverse *track*)

Note that if MAX-REVERSE-SAMPLES is too large, it will probably crash.