If you are using a Timeline, check the Audio drop-down menu. Under the Stereo mix-down option, select Turn-off. If there's a red bracket under output channels 1 and 2, it's an indication that the Timeline is getting mixed down to stereo.
Also in Edit>Preferences Speaker placement, at the bottom of the dialog, there's an option to perform a Stereo mix. If this is selected and you play a Sound with more than 2 outputs, the red bracket appears in the DSP Status to alert you that multiple channels are being mixed down to a stereo output.