dialogue coming thru wrong speaker (1 Viewer)

i had a couple of odd experiences lately with two movies. while watching American Sniper the dialogue suddenly starting coming thru the left rear speaker. it corrected a few minutes later then happened again. all other sound effects were okay. A bit later i was rented Jupiter Ascending and had the same issue except it was the whole movie, at least as far as i bothered watching. It hasn't happened with any other movies and the only thing they had in common was they were both Warner releases. Although i have watched a new warner release since that didn't have the issue. I was just wondering if anyone has had a similar problem or has some idea what could be causing the problem. Rob.