It can be progressively-muffled with this variation of Steve’s code in the Nyquist Prompt …
(setq start-freq 22000) ; cannot be greater than 1/2 the sample rate
(setq end-freq 200)
(setq passes 6) ; more passes for a steeper filter cut-off
(setq sweep-type 0) ; 1 for a linear sweep, 0 for exponential sweep
(let* ((nyq (/ *sound-srate* 2.0))
(f0 (max 0 (min nyq start-freq)))
(f1 (max 0 (min nyq end-freq))))
(if (= sweep-type 0)
(setf lpfreq (pwev f0 1 f1))
(setf lpfreq (pwlv f0 1 f1)))
(dotimes (i passes s)
(setf s (lp s lpfreq))))
https://forum.audacityteam.org/t/nyquist-code-for-gradual-high-pass-filter-please/30601/1
If you reverse the track then apply the code , (then reverse it back) , it gradually becomes less muffled,
similar to … https://soundcloud.com/glitchhop/amb-get-loose#t=2:33