Sounds can have different numbers of channels, and those channels can map to various speaker layouts.

mono 1 channel.
stereo 2 channels. The first channel is for the left speaker and the second is for the right.
ambisonic 4 channels. Ambisonic channels don't map directly to speakers but instead represent directions in 3D space, sort of like the images of a skybox. Currently, ambisonic sounds can only be loaded, not played.

See also