The Channel Mixer in Guitar & Drum Trainer lets you mix the left and right audio channels across and into each other simply and easily. You decide how much of each channel is played back in each channel.
For normal audio playback with 100% of the left channel played in the left channel and 100% of the right channel played in the right channel, settings look like this:

Reversing the playback with the left played in the right channel, and the right played in the left channel is done like this:

Which would sound "normal" if you were to stand on your head facing the speakers or the same as if you were turned with your back to the speakers.
The next channel mixer setting will effectively give you a mono mix:

While this effectively mono-mix setting will double the gain, which results in a volume increase of 2x the settings above:

By adjusting the playback position of the channels, you can change the sound stage.
Shifting the sound stage is useful when filtering out center panned audio and non-center panned audio from mixed music. If music is panned off center, you can sometimes bring it back into the center to remove it. Combined with the EQ, more filtering possibilities let you filter back in parts that were removed.