If it is just for fade-out or fade-in , e.g. see … Nyquist code for "gradual" high-pass filter please - #5 by Trebor
there’s code you can paste into Audacity’s “Nyquist Prompt” effect to do that ,
see the code in the posts … Nyquist code for "gradual" high-pass filter please - #4 by steve & Nyquist code for "gradual" high-pass filter please - #7 by Trebor