Sideskroll, you've got it backwards. Dolby Pro Logic and Pro Logic II are legacy standards for matrix encoding surround sound into standard stereo (and presumably analog, when the standards were created) signal, and likewise extracting from the same. It can be used over a digital medium or encoded into a digital recording, but the surround signal is still matrix encoded into the stereo waveforms. It isn't in a bitstream or a separate hidden channel. It doesn't get stripped out of the signal coming from the Wii/vWii, because it's still encoded inside a normal stereo signal. In order for that data to disappear, the console would have to actively look for the encoded audio, decode it into the extra channels, and then drop them. I'm pretty sure it doesn't do any such thing. Likewise, when you set PCM, your equipment is not "downconverting" to stereo. What happens is you are instructing your equipment to play the audio as normal wave data without any special decoding. Hence it leaves the stereo waveform (with matrix encoded surround still hidden inside) as-is instead of converting the stereo encoded sound back into surround.
