Correct, the older versions of PD with Wave Editor could do that, the newer PD versions with Audio Editor can not, basically lost functionality. If you happen to have AudioDirector with your PD18 you can do it there, or in most cases it's simply easier to use the free code Audacity,
https://www.audacityteam.org/ , for this audio edit.
Another option might be to launch your old Wave Editor as a standalone product if you still have it installed, read in a wav file extracted from PD18 video stream, do what you did previously and save track as a new wav file in Wave Editor and use that audio in PD18.
Jeff