Connect one channel to the audio component to which you want to feed a mono signal. Two-channel, stereophonic sound is by nature split into monophonic signals for left and right channels. Either channel output can be used as a mono signal. Note that some audio data from the other channel may not be audible.
Sum both stereo signals to mono by way of switching or post-processing. Many receivers and amplifiers have a mono switch, which combines both channels in a center-panned orientation. This will maintain all audio information into a mono signal. Audio editing software frequently has this capability as well. If your original signal is digital in origin, it can be processed to mono easily. Freeware audio editors are available (see Resources).
Split a stereo digital file using an audio editing software. Depending on the software package you use, you may have several ways to split left and right information. Some programs will offer a choice at loading the sound file to create a stereo track or two mono tracks. Summing is often available as well, or it can be created from two mono tracks by panning both to center, effectively superimposing them.