Well, well, well, I’ll be …
I re-read the last post from @shotcut and @djcorus and two things stood out.
Phase cancelation and “it worked with headphones”.
Went back to the original clip from @djcorus played it on my Android using headphones and what do you know…there is audio.
What the …?
Why when the built in speaker is used there is no sound but when there are headphones there is?
So decided to try a little experiment, extracted the audio from the original clip posted by @djcorus and imported it into Audacity (sorry @Hudson555x I know how much you like Audacity ) then did a mix down to mono, and what do we get? …
So Dan was correct, phase cancellation and the theory I had about the duration was wrong.
Problem solved I think.
A quick way for you to check if your audio is in phase is to apply the Downmix filter to it.
If the audio still plays it’s fine, if not then you will get cancellation.