I am using macOS Mojave v10.14.5. After I plugged in my headphones (while the MacBook is not plugged into power and opening KMPlayer to listen music on speakers), I was unable to get the sound output my headphone. Still the actual sound came out from MacBook Pro speakers instead of headphones. Any idea on this issue?
Are you sure KMPlayer is not overriding the default speaker setup? Have you tried going to Main Controls > Audio > Audio Renderer and setting your device as default?