Why does mp4 sound sound strange in Firefox + videojs player?

I am testing a 5 minute 36 second Mp4 video in videojs player in different browsers (Chrome, Firefox, IE-11 on Windows; Chrome, Safari and Opera on iOS). Firefox only has the following problems:

• After 42 seconds in the video, the audio stops and does not resume.

OR

• after 42 seconds, the audio stops and the last 30 seconds of audio plays (i.e. 5: 04s to 5: 35s), then stops and the video continues in silence.

NB: This happens in Firefox off and only with this mp4. Sometimes it works fine (both locally and off-server), sometimes it has one of the above problems. A common sign of trouble is that the video does not start downloading immediately, there is a wait of about 10 seconds; if it starts loading right away, the sound plays normally. All other mp4s I play with no audio issues in Firefox. If I open the problem file directly in the original Firefox player, there is no problem (but they were only tested twice).