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.