The Wii VC games technically shouldn't obey the Wii sound settings because each game is suppose to be a direct port which is ran on top of an emulator. To my knowledge, the hardware for most (if not all) of the consoles that are emulated on the Wii did not have a stereo/mono option.
Instead, the Mono/Stereo output was controlled and toggled from within each game. As an example, I seem to recall the Final Fantasy games for the SNES had these settings in their options from within the game play menu.
I never played Super Mario World much as a kid (didn't own a SNES :( ) but Nintendo has always tried to spot-light the quality of their new hardware by the release of a few games that integrated most of the new technologies provided by their new hardware.
I think the SNES was the first Nintendo console to use stereo sound. Since, SMW had stereo sound, I doubt there was a way to cut if off because Nintendo would have wanted to show off the systems audio capabilities.
SOLUTION
Sorry for taking so long to respond to this with, um, an actual solution but there is a hardware way to fix this.
Plug your left and right audio RCA cables (which I assume you are using) into an RCA splitter. You can then plug your one cable into your working TV channel and you will get both audio channels to play through the one speaker.
Here is an example of what you are looking for. As you can see, these things are dirt cheap, and if shipping isn't an option, you can probably find one in a local Walmart, Target, RadioShack (or international equivalent.)
I actually have a couple of these splitters and they were necessary in my childhood because I only had a mono-input TV in my gaming room. I actually daisy-chained some of them because I had a bunch of gaming systems as a kid and I didn't have a multi-input box. To much daisy-chaining will degrade the quality of the sound, but I'm guessing with your setup that's not going to be an issue.
The manual might give you the controls and a basic overview, but what you may want to look at is a FAQ/Guide or a Let's Play video walkthrough.
For instance, here's a Let's Play of SimEarth:
In addition to watching someone play the game with commentary and sometimes extra information, you'll get an idea as to how to complete particular challenges and where the tough parts are.
GameFAQs is generally a reliable site for game walkthroughs, although for this game they currently only have ones targeting the PC version. The changes might be minor enough not to make a difference, so you might want to check it out. A game FAQ is generally going to have more detailed explanations of the game mechanics, and less "funny" commentary which you might not be interested in.
Best Answer
If the code is for the Wii U, then it will only work for the Wii U, regardless if the game is on both the 3DS eshop and the Wii U eshop. So no, it won't work for your 3DS if you have a WiiU code.