I think I've seen this issue before, and I'm only submitting mine as there might be some additional, useful, information.
Problem: Audio track is muted during playback, regardless of volume level in VLC.
Workaround: Use Airplay button to switch to internal speakers. Video will pause. Hit play, then switch back with headset with same button.
Caveat: This problem will happen again on the next video.
Headsets affected:
- Trekz Air / No AAC or AptX - Happens as described.
- Sony WH-1000XM3 / AAC, aptX (+ HD), LDAC - Usually does not happen. When it does, simply rewinding 10s fixes it.
In addition, I've noticed this issue come up on videos with AAC and one with a DTS track, but not AC3 tracks. Not sure if it's related. I've got a couple more headsets that I will test with later and update this post with.
My hypothesis, which could be and probably is terribly wrong, is that iOS is sending AAC audio to headsets that don't actually support it, resulting in the muting. I also don't recall this issue coming up on a pair of Beats X headphones I own, but I won't be able to test them until tomorrow.