Missing speech at a certain part of video

Question

new here and to video production in general.

The issue I have is audio-related.

Background: I have this video clip made by a vendor - it has background music, speech, visuals, etc. The video clip plays fine with no missing audio on laptops/desktops/mobile devices.

Problem: When the playing device, e.g. laptop is plugged in to a projector and speakers via HDMI, some speech in the video clip disappears (i.e. cannot be heard). The background music and such is still present, as is speech in other parts of the clip. There are no problems with the projector and speakers AFAIK, as they have been used to play other videos with no issue.

Does anyone have any idea what the specific problem may be that I can feedback to my vendor to look into? Thank you.

(Apologies if this is not the right place to ask this question)

Answer 1

This is almost certainly because that section of the speech has been recorded as a stereo pair of files, but one of the files is out of phase with the other (if you looked at the waveform it would appear is upside down compared to the other channel's waveform).

If you play these two channels out of a mono loudspeaker (which many projectors have), the two channels will cancel each other out, and you will only hear the background sounds which are correctly in phase.

More here: Premiere Pro audio muffled and distorted on export