$ ffmpeg -i audio.mp4 -vn -an -codec:s:0.1 srt foo.srt
$ cat foo.srt
1
00:00:00,000 --> 00:00:00,000
<font face="Serif" size="18">Mary has a little</font>

2
00:00:00,038 --> 00:00:00,038
<font face="Serif" size="18">lamb.</font>