Wii:Audio Information

Surround Sound
The Wii made extensive use of Dolby Pro Logic II surround sound, effectively identical to the way the GameCube did.

Dolby's Pro Logic II standard extrapolated a 5.1 surround mix from two audio channels. Each of the two stereo channels carries an inverted-phase signal that become the surround-left and surround-right, with the center channel being made up of any sounds that are mono (panned dead center between left and right), such as dialogue in cutscenes. The ".1" in "5.1" refers to the sub-woofer, where all sounds below a certain frequency are routed to. Many PS2 games have a Surround option in their audio settings, but some only list "Mono" or "Stereo", despite boasting the Dolby Pro Logic II logo on the packaging or disc label. Generally in this case, the stereo option serves double duty as both a stereo signal and a 5.1 Pro Logic II signal when decoded appropriately.

Wii games played through a Wii U via HDMI will send a digital audio signal via HDMI and can still be properly decoded if the receiver knows to be looking for Pro Logic II.

Some modern AV Receivers are quietly dropping compatibility with older surround formats, but others are holding strong. It is recommended to research the surround format capabilities of a given receiver before purchasing new.