Audio mute to video and mixing audio file with FFMPEG

[mikoyuri]

I am trying to add a piece of mute on a video file and to mix an audio file into the video that I want it starts at 20 seconds, it works, but something strange happens the audio duration of the output file is slightly shorter than the original, this is what i use:

ffmpeg -i video.mp4 -i audio.wav -filter_complex "[0]volume=enable='between(t,10,15)':volume=0[v];[1]adelay=20000[ad];[v][ad]amix=inputs=2[m];[m]concat=n=1:v=0:a=1" out.mp4

I leave in attachments the original video waveform and output waveform

What I am doing wrong?

Thank you.

[ProWo]

Try

ffmpeg -i video.mp4 -itsoffset 20 -i audio.wav -c:v copy -c:a pcm_s16le -af "volume=enable='between(t,10,15)':volume=0" -map 0:0 -map 1:0 out.mkv

(pcm in mp4 could be problematic)