It depends whether your Logitechs have an optical input and a Dolby decoder built in or not. Don't know because we don't know what speakers you have.
IF the speakers have an optical digital input, then get an optical cable with appropriate ends (check first) and let the speakers decode the signal. You will NOT be able to control volume through the Mac software.
IF the speakers have analog inputs only, you will have to get an external 5.1 USB or Firewire audio output device. Or run them in 2.1 from the standard line outs.