[FFmpeg-user] Add audio at specific time in video

[Jay Sardhara]

I want to add a *.m4a audio of 5s duration to a *.mp4 video file. I have
already tried the -itsoffset command of ffmpeg.

ffmpeg -i a.mp4 -itsoffset 00:00:10 -i b.m4a -map 0:0 -map 1:0 -preset
ultrafast o.mp4

The problem is it does start at 00:00:10 but it keeps repeating after every
10s. (It is noticed only when you play the whole video w/o touching seekbar)

Also other problem is that when I play this video on android then audio
starts from beginning. While it plays fine with VLC.

*Expected O/p* Silence in video till specified time, then my audio, and
again silence after audio
How to achieve above result ?