You may choose to mix the sounds to one single monophonic channel, or to mix them to a stereo pair of channels.
At the left of each channel, there are two small icons representing speakers. By clicking them, you can turn on or off the output to the left or right speaker for that particular channel.
When mixing stereo, use these buttons to control how each channel will be mixed: if it is set to play through the left speaker, it will be mixed to the left channel; similarly for the right speaker. If it is set to play through both speakers, it will be mixed to both channels.
If you check the “Use each channel’s pan” option, the volume of each channel will be taken into account when mixing.