Try this as the last plugin on you master to sum to mono.
http://www.kellyindustries.com/stereo_tools.html or http://www.plugin-alliance.com/en/plugin..._solo.html with the width nob turned to zero.
Both are free.
http://www.kellyindustries.com/stereo_tools.html or http://www.plugin-alliance.com/en/plugin..._solo.html with the width nob turned to zero.
Both are free.