hi wakiki,
You need the Common, as its the common return for the right and left
audio channels. So you need something like a screened pair cable.
One wire of the pair to the right channel and the other wire of the pair
to the left channel. The screen of the cable is connected to the common.
If its important to your project, make sure the right and left channels
do not cross over, otherwise the left channel sound will come out of the
right speaker.
Got the idea.
Let us know.
Regards
EricG