ffmpeg!
ffmpeg -i in.wav -af "afade=d=0.25:curve='qsin', areverse, afade=d=0.25:curve='qsin', areverse" faded.wav
this horrible hack reverses the file, applies input fade, reverses again, so you don’t need to know the duration.
(sox syntax for fade-out is smarter but tbh i have never really learned it.)