How to change audio frequency?

With ffmpeg:

ffmpeg -i <input> -af 'asetrate=44100*1/2,atempo=2/1' <output>

Here, 1/2 is the pitch factor. See the other answer for more details.

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If you need a GUI, use Audacity, it's a free, open source, cross platform audio editing tool.

Features: Change the pitch without altering the tempo, or vice-versa.

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As an alternative, try sox. Something like that:

sox <infile> <outfile> pitch <shift>

where gives the pitch shift as positive or negative ‘cents’ (i.e. 100ths of a semitone). There are 12 semitones to an octave, so that would mean ±1200 as a parameter.

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.

References

• https://www.ffmpeg.org/ffmpeg-filters.html#atempo
• https://www.ffmpeg.org/ffmpeg-filters.html#asetrate