Why does this video's audio change depending on the playing device? Yesterday I noticed that if I played this video from my smartphone on speaker, one could barely hear anything. On the other hand, if I used earphones I could hear it perfectly.

Then, I tried with several devices (other phones, my PC's speakers, a Bluetooth speaker) and on some of them one could listen to the audio with a really high quality, but on others it was very low and like 'echoish'.

I believe there should be a signal processing explanation to this phenomenon. I don't really understand what's going on, but I suspect it has something to do with stereo and mono audio. Sound quality was good on those devices that were stereo (speakers from PC, earphones, headsets) but on those that were mono (speaker of a cellphone, Bluetooth speaker) I could barely hear anything.

What is really going on? Is this phenomenon a mistake from the video uploader or could it be an intended effect?
The 2 stereo channel might be out of phase (near opposites, thus cancelling each other out when mixed to mono).

The human ear-brain doesn't just sum stereo left-right to the 2 ears, but instead uses any phase differences to help determine directionality.