Which mode is most commonly used for voice communications on the 160-, 75-, and 40-meter bands?

For single sideband (SSB) voice communications on frequencies below 9 MHz, amateur radio operators conventionally use Lower Sideband (LSB). This encompasses the popular 160-meter, 75/80-meter, and 40-meter bands. The reason for this division dates back to early radio manufacturing. Using a common 9 MHz intermediate frequency (IF) filter combined with a local oscillator resulted in a mathematical inversion of the sidebands depending on the band. It was simply cheaper and easier for engineers to output LSB on lower bands and USB on higher bands. Today, we continue to observe this band-plan convention to ensure everyone can readily communicate. If you transmit on USB while everyone else on 40 meters is listening on LSB, your voice will sound like scrambled "Donald Duck" audio to them.