Low audio through headphones when viewing video especially speech

[Randy baton]

Hi

I was just trying to watch film on a plane and could hardly hear any of the talking, however the background noise and music in the film sounded fine. I thought it could be something to do with the film being in 5.1. But I also get distorted / low speech on a stereo film. If I listen via the tablet speakers it sounds fine.

Any ideas?

Is there any way to force all headphone audio into mono?