Well, really you can't. You can only combine the L and R signals. Or you could reset all the pans to center but that's a lot of trouble.

My old stereo receivers have mono buttons. On a PC I'm not sure how you do it internally. On my work PC I go to control panel, sound and audio devices, audio, advanced, and select mono laptop speakers. Whether that affects the headphone jack I'm not yet certain, but it sounds like it does.

I've seen pro and con discussions before about making backing tracks in stereo to go into a PA. A lot of your audience will not be in the center of the speakers so one line of thinking is to go mono.

After a little investigation I find there's a growing cult of people who prefer listening in mono.

Wow, until yesterday it'd never occured to me. I was looking for a small mono hi-fi amp or kit and ran across mono-stereo discussions. Have an old Victrola that I don't have to time to restore properly so thinking I might fit a mono amp, one speaker, and an mp3 playing discman into it.

Now everything I was sure about listening to music is suspect.

Like the man said, "ain't that a kick in the head!"