The Quick Mix effect mixes the selected sounds.

You can mix the sounds to one single monophonic channel, or to a stereo pair of channels.

At the left of each channel, there are two speaker buttons. By clicking them, you 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; if it is set to play through both speakers, it will be mixed to both channels).