How do I swap audio output of the left and right speakers?

You could get a cable that splits into two channels and one that merges them again. Then just swap the channels inbetween. For example this kind and this kind, if I read correctly... cables and audio isn't my thing :p Anyways, something along those lines should work maybe work?

There is a software solution called Equalizer APO

• configure it to attach to your output device (Usually something like: "Speakers - VIA HD Audio" or "Speakers - Realtek HD Audio")
• create a new file in C:\Program Files\EqualizerAPO\config, for example swapchannels.txt
• Paste the following line into it as its only content: Copy: L=R R=L, save the file
• Open Equalizer APO's configuration editor, make sure that your device is the correct one in the dropdown menu on the top right
• If applicable, remove all pre-configured options by clicking the red "minus" icon
• Click the green "plus" icon to add a new configuration for your output device, then navigate to the "Include configuration file" option, select your swapchannels.txt file
• Save/Apply the configuration. Left and right channel are swapped.

Source of tutorial here.

For my own case I ended up using headphones. Mostly left and right do not matter for Windows sounds or even not for music that runs in the background. However, for FPS games it matters a lot ;)

Some soundcard drivers actually have an option to swap the left and right channels in their Control Panel applet (eg. A Realtek adapter I used to have did). I imagine this might have to do with the features of the specific sound chipset. Other drivers/chipsets don't.

Some games/applications have software methods to swap the left and right channels in their settings/configuration window/file (eg. Final Fantasy VII PC, Winamp). Others don't.

The only guaranteed-possible way to swap is to physically swap the wires/speakers in hardware.