This is not a technical question nor is it about surround sound in Wii U games (Surround sound is working fine in Wii U games which support it). This is an issue with surround sound in Wii mode only.
On the original Wii, there was a configuration setting that allowed the user to choose between stereo and surround sound. This setting has been removed from Wii mode on the Wii U. Instead, Wii mode uses the Wii U's configuration setting for sound. None of the Wii U's configuration settings will enable surround sound in Wii mode. If you select surround on the Wii U, in Wii mode it will output 5.1 channels of PCM audio and receivers will only play audio in two channels: left and right. The remaining channels will be silent. If you select stereo, in Wii mode the WIi U will output 2 stereo channels without the embedded surround matrixing (this is equivilent to the original Wii's stereo mode).
Some Wii games such as, for example, Resident Evil 4 have an option in their menu to enable surround sound. This is equivalent to the original Wii's surround setting and surround sound will work if you have your Wii U set to output stereo (and not surround). It will only, however, work for those particular games with such an in-game setting. Most, if not all, Nintendo-published titles do not have this setting in-game (they read the setting from the Wii itself which in the Wii U's case lacks the capability of choosing surround for Wii mode).
This has been brought up before, but I thought I would bring it up again to make sure it is still "out there". It would be great if next week's update addressed this, but I won't hold my breath since it's probably low on the list of priorities (if it's even on such a list).
Because surround sound can be confusing to some, I just wanted to clarify how it works on the original Wii compared to how surround works on the Wii U (for Wii U games, not in Wii mode which does not work, as explained above). The Wii U is capable of outputting six channels of discreet audio in Wii U games (commonly referred to as 5.1 where the sixth channel is the .1 which is the LFE/subwoofer channel) via HDMI. The original Wii is not, the original Wii achieves surround sound through a matrix process which embeds the surround information into a stereo signal (stereo is two channels of discreet audio, on the Wii output via red and white RCA-type cables). The surround receiver will then "unfold" (or decode/dematrix/etc.) the embedded surround channels to re-create a 5.1 surround signal. Since the surround signal comes from a stereo (two channel) signal, it will never be as good as a true multichannel signal like the Wii U can output in Wii U games that support it. But the games still sound pretty amazing, especially Super Mario Galaxy 1&2 and Zelda: Skyward Sword. They sound flat when played on the Wii U since Wii mode does not support it.
The issue is specifically with the matrixing information which on the original Wii, again, was enabled by switching from stereo to surround. And again, this setting has been removed from Wii mode on Wii U. And also again, some games such as Resident Evil 4 have the ability to switch it on in their options menu. Most games, however, do not.
Since I came across some misinformation on the internet, I just wanted to address that. No, switching the Wii U to stereo will not allow Wii games in Wii mode to play surround sound. It will, however, allow some virtual console titles (SNES and N64 games) which support dolby surround to output surround sound which also uses a matrixing process, but an older one. Wii games (like the gamecube before it) use the more recent dolby pro logic II for matrixing. The difference is the original (the one some VC games use) was only capable of producing 4.1 channels in a stereo signal while pro logic II (which the Wii uses for surround in games) allows for 5.1. The other difference is that for whatever reason, it was "safe" to include the matrixing information in the stereo signal which is why it just works in those VC games. For pro logic II, at least with Wii games, that must not be the case because it is only enabled by switching from "stereo" to "surround" in the menu setting, which again does not exist on the Wii U for Wii mode.
This is not a licensing issue since the license is for authoring, not for playback. You can put your Wii U in stereo mode and let your receiver "make up" fake surround information which is what it will do in pro logic ii mode when receiving any stereo signal. Thiswill not produce a true surround effect. For example, when Mario is flying around in Mario Galaxy, the rears will never be active like they were in the original Wii's surround mode. I still have my original Wii hooked up just because the surround is that important for me.
I don't see why this isn't an easy fix. If the user has "surround" selected under the Wii U's menu, when Wii mode is booted up it should switch to a 2.0 stereo signal with the matrixing option enabled to output surround sound in Wii games. Right now it remains in 5.1 (which the Wii does not support) with only the left and right channels active. I do not expect the Wii U to 'dematrix' the surround information into a 5.1 signal. I'm not aware of any non-PC consumer device which does such a thing. But I don't see why it can't switch to a stereo signal with the embedded surround matrixing enabled (when entering Wii mode) so we can enjoy surround sound on Wii games which support it. And BTW, using the red/white RCA plugs has nothing to do with it. Nor does switching to them enable it either. They also cannot be used if you're using HDMI. HDMI has nothing to do with it either, this is not a technical problem or a setup issue. It is a Wii U issue that can only be corrected on the Wii U. I have a feeling the next update isn't going to fix it 