Assuming that you only need to process a short bit of mono audio, probably the easiest way to do this sort of thing is to copy the sound data into an array, manipulate the array in whatever way you want, then convert the array back into a sound.
If the selection is more than a few hundred thousand samples, this code will probably crash, but it’s a good solution for short selections:
(setf ln (truncate len)) ; the number of samples as an int
;; Copy the entire selection into an array
(setf my-array (snd-fetch-array *track* ln ln))
;; Convert the array back into a sound
(snd-from-array 0 *sound-srate* my-array)