How can I unify my stereo audio output to make it mono?

I use stereo speakers or headphones, and for "accessibility-related" reasons, I need to have the same audio play from both sides. Now, I don't need this for some specific audio file - which I could edit to downmix to mono; I need this for the audio output of my Linux system. How can I make this happen? System info: * Devuan GNU/Linux Daedalus * Cinnamon desktop environment 5.6.7 * Linux kernel 6.1.0. * Using USB headphones at times, on-board audio analog output at other times. Additional complication which you can tackle if you'd like, or just ignore: I would like to have this happen only for the USB headphones.