Why is my YouTube video's audio broken on 20-50% of mobile devices?

[Kaos-Industries]

Hi,

A few hours after I uploaded an FFmpeg-encoded YouTube video, the few people that I had sent it to started reporting a strange issue: that the first 26 minutes of the audio sounded very distorted; some have described it as sounding like it's underwater. Here's an example of what it sounds like.

I then started testing with more people, and came up with the following data from a total of 31 mobile devices.

Normal Audio:

Galaxy S10 (x2)
Galaxy S20 (x3)
Galaxy A50
iPhone XR (x3)
iPhone 7
iPhone 8 (x2)
iPhone 10 Pro
iPhone 11
iPhone 11 Pro
iPhone 11 Pro Max
iPhone 12 (x3)
iPhone 12 Pro
Oppo F11
OnePlus 7T
Pixel 3A XL
OnePlus 7 Pro

Distorted Audio:

iPhone 6S
Galaxy S7 Edge
Galaxy S8+
Galaxy S10 Lite
Huawei Y6 2019
Huawei P30 Pro
Honor 10 Lite

The audio for those first 26 minutes is broken for 7 out of 31 mobile devices, so 22%, and this increases to more than 50% when only Android phones are included. Most of those I asked were based in developed countries, which means that the data also skews towards newer, more modern phones, which means that figure could easily be closer to 80% in less developed countries that primarily use older Android phones.

Needless to say, this is a pretty big problem for someone like me who expected to be able to upload to YouTube and let it handle the rest instead of having to do web-dev-like device debugging.

I compiled the video from several source videos, encoding and concatenating them using FFmpeg. It uses H264 and OPUS as codecs, but more relevantly, checking the source video that those first 26 minutes comes from shows that this problem originates in that video, so now the question becomes:

a) What is it in that particular source video that is causing the audio to become garbled in such a large proportion of mobile devices?

b) How can I fix it, either using FFmpeg or anything else?

I would really appreciate some help with this as I've spent a long time on this project, am very much out of my depth and all I want to do is make sure my YouTube upload can be seen by all of the people who want to.

Thank you.