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.
Best Answer
There's no way to get a true widescreen mode. Like Rapitor said, when these games were originally released as Nintendo 64 cartridges they only supported the standard SD 4:3 apect ratio, as that's the only aspect ratio TVs came in back then. The game you bought is the original cartridge version running under a emulator (the "Virtual Console") so the game has all the limitations of the original. It's not possible to support a true 16:9 widescreen presentation without updating the game in some manner to add that feature. For that you'll have to wait for a hypothetical HD remix version of Donkey Kong 64.
In the meantime just play the game. It won't be long before you stop noticing the black borders. Alternatively if the black borders really bother you, but squashed characters don't, you should be able to get the game to fill (most of) the screen by setting the Wii U to output 4:3 and having your TV stretch it to fill the 16:9 display.