Answer by llogan for How to remove last 4 seconds of MP3 file?

Adapted from Cut video with ffmpeg:

ffmpeg -i input.mp3 -ss 4 -i input.mp3 -c copy -map 0:a -map 1:a -shortest -f nut - | ffmpeg -y -f nut -i - -c copy -map 0:0 output.mp3

What this does is uses the same input twice in a roundabout method to trim without needing to know duration and without re-encoding.

1. The second input has 4 seconds cut off from the beginning.
2. The -shortest option uses the shortest input duration to determine duration of the outputs. This results in cutting off the last 4 seconds from the first input to match the duration of the truncated second input.
3. Because both streams are needed for -shortest to work they are piped to another ffmpeg to remove the second input.