It's important to note that
stereo plugs (2-channel) are distinguishable from
mono plugs (1-channel) by the number of
black stripes on the end of the
plug itself. If it has a thick and
thin line (thicker towards the end), it's
stereo. If it has only one line, it's
mono.
If you want to bring a source from
stereo to mono, don't try to use a mono plug, because you will often only get one of the two channels (for instance, only left), instead of a mix of both. Instead, use a specialized adapter, something you can probably find at
Radio Shack or some similar electronics or
audio hardware store.