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I have a client who did a short video on their own and ended up doing the same thing. The clip sounded fine on his computer but almost no sound when posted online. The reason it sounds fine on your PC is that you are listening on a stereo setup. Two discreet audio feeds are being sent to separate speakers. At the showing, the PA setup was mono. They summed the two audio channels into on signal path and the two channels canceled each other out.
Think of it this way… an audio signal is a wave. If both channels are phased correctly, the two channels will simply be added together with no decernable effect. If the two are out of sync, the equal but opposite signals will cancel each other out. 1-1=0. Capiche? The problem is figuring out where one of the signals got reversed. It could have been done when you edited the program. It could have been a impropperly wired cable at your client's presentation or the audio guy could have reversed the phase on one channel on the pa system. It's pretty easy to hear out of phase audio. Sit equally between the speakers, send the same signal to each and if the phase is reversed, it's almost like the air is being sucked out of the sound. It almost hurts a bit. There are plugins, meters and all kinds of gadgets to check phase but your ears work just fine once you learn. As for the PA guy at the event, he was an idiot. Anyone worth their salt knows that the quick fix is to unplug one channel.