For the last hour, I haven't been able to get sound to come out of my external speakers, but it was working fine with the HDMI speakers on my monitor.

I have been making myself crazy, trying to figure out what the heck I did wrong, what config files I needed to edit, which libraries and plugins I needed to reload, etc. I've restarted my computer a bunch, and began to consider reinstalling Mint (I run Mint 19) ... then I realized that the volume knob on my speakers was turned down to zero.