Sega Genesis/Mega Drive Model 1 had a Headphone Jack. The Genesis/Mega Drive was made this way for a reason. You could either put in Headphones and listen to the game in stereo while sound still came out of the TV in mono at the same time or you could hook stereo speakers into the headphone jack. I've also hooked in a sound splitter and have done both simultaneously!
While some of you may know this already, the Genesis/Mega Drive was programmed to automatically output to stereo when Headphones were inserted into the Headphone jack. That programming is still there in later models but all of them except two don't have a headphone jack so they're unused. (The other two that do are the Genesis Nomad, a hand held Genesis model 2 and the Sega CDX/Multi Mega which is a Sega CD/ Sega Genesis/ Portable CD Player.)
Now to take advantage of the Mono Sound coming out of the TV and the Stereo Sound coming out of the headphones/Speakers simultaneously, the trick is to have a volume control on the Headphones or speakers. (like a volume Slider.) If you balance the volume slider out into the middle or so and
turn the TV volume up to a certain point (Depends upon your TV's Volume settings. Mine works well at Volume level 22) It will give sound effects the feeling of depth. voices will sound like they're in the room with you as will sound effects. This is actually just an illusion created by the mono sound from the TV and the middleways volume on the Headphones/Speakers. It works for any Genesis/Mega Drive games as well as any of its add-ons.
This is also how to take advantage of Q-Sound.