Speaker not playing back FCPX properly. Plays itunes OK so its not a speaker problem

[Tom Brooks]

Same problem has cropped up in new installation of macOS Sierra, FCX 10.3, Kona LHe+. Fix is about the same–maybe a slight variation. Go into Audio/Midi Setup, change Speaker Configuration to something other than Stereo and make sure you apply the change. Then change if back to Stereo. Seems to fix the issue.