Sounds like you have some step of the way from game to output that virtualizes surround sound and fails to integrate the center channel.

Try following this very beautifully drawn flowchart and make sure your speakers is set to off in that tab.

[Linked Image from i.imgur.com]

If that doesn't work, try to consider if you have any apps or similar that has virtual surround features, such as some logitech 5.1 virtual headsets that maybe is interfering with your speakers and turn those off.