We have been sent a sample DVD of someones work.

On first play it was obvious that the voice-over was hardly audible over the music and ambient sounds.

I played the same DVD through a different set-up and the voice-over was at the volume that I would expect.

I had this exact occurrence some five years ago but the DVD was returned before I got to the bottom of the cause of the problem. Although I strongly suspected an out-of-phase of the voice-over.

We will be analysing the whole system to try to understand better what is the cause and hence solution.

Any ideas?

Regards
Martin

99% of the time this comes from using "stereo" tracks for the dialog (or voice-over or whatever).
Under certain (unfortunate) conditions you can end up with what ought to be a monaural track (from a single microhone) recorded to TWO tracks ("Left" and "Right" "stereo") exactly out-of-phase. So, if you listen to the final mix in "stereo" ti will sound reasonably "normal". But if you play it back in monaural, those out-of phase tracks will almost completely CANCEL each other out, and you will get exactly the symptoms you are describing.

The solution is to NEVER NEVER EVER use a "stereo" recording of a single microphone. Pick EITHER the Left channel, OR the Right channel, and immediately DISCARD the redundant (and potentially out-of-phase) track. If you do this faithfully, you will never have this problem again.

Just a guess...

The "sometimes" thing might have to do with systems having a center channel. When it's played back on a two channel system, the signal will be low in the sweet spot and will sound relatively normal when walking around. If the audio is routed to the center channel of a 5.1 system, it will be cancelled electronically, rather than acoustically, and will be eliminated no matter the listening position.

...or, it sounds great on systems where the front channels happen to be wired out of phase. :)

99% of the time this comes from using "stereo" tracks for the dialog (or voice-over or whatever).

This. Famous cause of head scratching.