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: