3D Spotlight has updated their Speaker Setup & Soundcard Tweak guide.

Virtual 5.1. This setting is available when 6 Speaker mode is selected. When this setting is Checked (Turns Red) it enables the use of a virtual/synthesized Centre (Comprised of the Front Left/Right output) & Subwoofer (Comprised of all out channels except the
Centre) channel in Applications/Games. In most instances it is recommended that you Uncheck (disable) this feature if the Applications/Game you wish to use supports 3D Sound API´s, such as EAX (& you wish to use them), as it can distort the positional effects
offered with the use of these API´s. As such you may want to experiment with this setting, or only enable it with certain Applications/Games, generally those that do not support 3D Sound API´s (Or you have disabled their use). Another alternative would be to mute the Centre speaker, this will allow the enhanced Subwoofer content to be played, while not effecting the overall positional audio.

Also if you intend to use 6 Channel supporting Software DVD players (e.g. PowerDVD & WinDVD) or Multi-channel wave files you should also disable this setting, because there is no need to synthesize the
Centre or Subwoofer channels.

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