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.
If the corruption problem was caused by the hardware, then the Wii will still corrupt those specific GameCube memory cards today, as there have not been any substantial hardware changes to the Wii since release - and the hardware changes that have occurred were primarily to prevent hacking the Wii.
If the corruption problem was caused by software, we don't know if it is fixed or not, as Nintendo has never acknowledged the issue, nor specifically said that a new version of the OS has fixed it.
If you are concerned about it, just buy a different card.
Best Answer
The Nintendo channel provides downloadable demos for the DS (like a download station you'd see in a GameStop or so). The WiiWare shop area in the Shop Channel supposedly has some downloadable demos, but not a whole bunch (if any). Nintendo is fairly stingy about demos...
Another option may be to just install the Homebrew Channel and whatever homebrew applications you feel like trying out.
Other than that, the Wii isn't very free-gamer friendly. A $20 Wii card will net you 2 to 4 games off the shop on average, though. Might be worth the investment, even if you're trying to play on the cheap.