If you listen to music on your Mac while at work, you may use earbuds so as not to disturb co-workers; however, this does have a tradeoff. When using both earbuds, you can drown out others who might be trying to get your attention, but if you only wear one earbud, you will miss the parts of stereo recordings on the channel for the bud you left out, which in some cases can make a dramatic difference in the sound you're hearing.
You can avoid this by setting your Mac to use Mono output. This will combine all audio channels into one, so all parts of an audio file will be heard, regardless of the settings of your audio player, music files, and speaker setup in the OS.
To do this, go to the Universal Access or "Accessibility" system preferences. In the audio section you will have an option to play stereo audio as mono. Check this box, and your audio will now come out as mono on any attached speaker or headphone system. For folks who use iPads and iPhones, this can be done by going to the General > Accessibility section of the iOS settings and enabling Mono Audio in the Hearing section.
Just remember when you use both headphones again to uncheck this option so you can have proper stereo output.