How do I force single (only) channel audio in youtube/flash videos to play in both (left/right) channels?
I just accidentally found a lame and hacky but working way to solve the issue - plug the headphones halfway, instead of fully, to the port :)
Windows 10:
Windows Settings > Ease of access > Audio, and on the right panel just toggle "Turn mono audio" to "ON".
Original answer now obsolete in updated Windows 10: Windows Settings>Ease of access> Other options> Audio options: Turn mono audio "ON".
Free, no additional software required for most: Use any basic "vocal cancellation" or "Headphone Virtualization" feature. It is offered by many audio drivers, e.g. with the standard Realtek driver, presents in most Windows systems:
Right click on the loadspeaker symbol in taskbar, click "Playback devices". Right click on the active audio device, go to the tab "Enhancements". Check "Immediate Mode", uncheck "Disable all sound effects" (Already default). Check "Vocal cancellation" or "Headphone Virtualization".
(Works for all audio with sound only on the right or left channel. Reason: Simple voice cancellation works by duplicating and inverting the audio of one channel and adding it to the other channel, resulting in a cancellation of the central audio. If only one channel has sound, we just get the desired duplication.)