ffmpeg -i input.mp4 -af "volume=<volume_level>" output.mp4
input.mp4: The input video file.
-af: Stands for audio filter. This option tells FFmpeg to apply an audio filter.
"volume=<volume_level>": This is where you specify the volume adjustment. <volume_level> can be a positive or negative number, where a positive value increases the volume, and a negative value decreases it.
output.mp4: The output video file.