Find input audio rate beforehand thus:
ffmpeg -i input.mp4
Assuming input audio rate 44,100 Hz, this command will do the job:
ffmpeg -i input.mp4 -af asetrate=44100*3/4,atempo=4/3 output.mp4
The factor of 3/4 will change most female and “skinny” (chipmunk) voices into male and “fat” voices. Use 4/3 for the opposite:
ffmpeg -i input.mp4 -af atempo=3/4,asetrate=44100*4/3 output.mp4
Notice reversed filter order to prevent signal degradation. Whenever possible, lossless operation should come before lossy operation. I’m not 100% sure whether I’m not making some mistake here from misunderstanding FFmpeg filters.
FFmpeg filter asetrate should have a variable named ir for input audio rate, in analogy to iw×ih in some video filters, but I couldn’t find any mention of it in the documentation.
For factors greater than 2 (such as 4/1 or 1/4), you must use multiple atempo filters (1/4 = 1/2 * 1/2 or 4/1 = 2/1 * 2/1):
ffmpeg -i input.mp4 -af asetrate=44100*4,atempo=1/2,atempo=1/2 output.mp4
I don’t know how to obtain “skinny” male voice and “fat” female voice.
Instead of -af, you can write -filter:audio or -filter:a.