Mono sound won't play back on movie made in iMovie.

I've searched the internet including Apple's imovie support forum but I can't find any answer to this. So I was wondering could anyone here help as you all helped me on other issues I've had!

I made a movie using imovie and some sound was recorded on a mic in mono. I put this in over the video and put some stereo background music over it.
I then burned this to a dvd using idvd.
The movie worked fine when I played it back off imovie and quicktime on my mac and its external and internal speakers. It also worked using a fairly old dvd player and also when I played the dvd on my macbook. Therefore I though the dvd was fine but I gave a copy of it to someone else and they said that they couldnt hear the voices which were recorded in mono, only the stereo background music. They said they were playing it on their mac but didnt say what model. I though this was really weird and I'm still confused about what caused it.

Does anyone have any idea what caused this and how I could fix it?
I'm more interested in what caused it so that I can stop it happening again next time.

Thanks