If your system has an optical input, get yourself an optical cable that has a mini-TOSlink adapter. You'll plug the adapter into the headphone output of the MacBook and then the other end of the optical cable into the sound system.
If it has no optical input, but a coaxial input, then you'll need the mini-TOSLink adapter (still, for the Mac) and then a TOSLink to Coaxial adapter.
If you have NO digital inputs, then you'll need a "Y" cable with a stereo minijack (headphone jack) on one end and the red and white (right and left) RCA connectors on the end.