Force subtitles to play as default
You can use MP4Box 0.6.2-DEV-rev453 (May 2016) or higher to do this:
mp4box -add alfa.mp4 -add bravo.srt:txtflags=0xC0000000 -new charlie.mp4

Example
This will mark the subtitle stream in the output file as forced.
However, this mark will only be recognized starting with these versions of
programs:

MPC-HC 1.7.10.207 (May 2016)

VLC media player 3.0.0-20161101 (Nov 2016)



I have seen mentions to this post on the FFmpeg mailing list about a
patch that implements disposition for FFmpeg:
ffmpeg -i alfa.mp4 -i bravo.srt -c copy -c:s mov_text -disposition:s forced charlie.mp4

However after having tried it with both &quot;forced&quot; and &quot;default&quot;, the subtitles
marked by FFmpeg are not recognized as forced by either MPC-HC or VLC.