For watching TV programs on your TV or listening to music from a stereo, I agree with pepst that headphones with a switch would most likely be the best solution.
For watching movies on your TV using a standalone player, many have the option to change to mono in their menu. I believe some TV sets have this option as well.
For a software solution watching or listening to media files on your PC, most software like PowerDVD, Media Player Classic, and VLC have this feature built in.
Using Media Player Classic: Play/Audio/Options/Audio Switcher
Using VLC: right Click screen/Audio Channels/(choose “right” or “left”)
PowerDVD also has this ability but I don’t recall the path (but it’s quite easy)
I hope I understood your question correctly.