okay thanks for the reply and your clarification
Your MacBook Pro has a combination audio output jack--headphones and optical (digital) mini toslink.those are your choices. IF you want digital audio out from your Mac, you'll have to get a mini Toslink / Optical Mini adapter, and a standard Toslink Cable. Shut Down your MacBook Pro, attach the mini adapter to one end of the Toslink Cable and plug it into the headphone jack of your MacBook Pro. Start your computer up. After it's running, plug one end of the Toslink adapter into the Digital Input on the back of your Logitech speakers, assuming that there is one. It should look something like a Capital Letter D lying on its back. Go to the Apple symbol at the top left of your screen, go to system preferences, then go to Sound. Change the sound output to Digital
rather than "Line Out" or "Internal Speakers". Play something. You should hear sound coming out of the speakers. If you have a set of Logitech speakers with the remote input box/volume knob, the one that's connected to the subwoofer by a large cable, then you would have to switch the input on that back to 2 or 3, whichever one the digital input is plugged into. The other one, which has an RCA jack on it, is SPDIF, not toslink. There are converters out there if you don't have enough inputs.
IF you look at the back of your Logitech Speakers and you see 3 -3.5mm sockets and nothing else, then you have 6 channel analog (no digital at all)
in that case, Diamond Multimedia makes an okay 6 channel analog external sound card which should work okay. you may be also able to get one from china that could work okay, maybe Mocha HIFI or HD rush...digital to analog converter . The problem is that 5.1 sound from your Mac requires more work especially for external hardware and the software needed to drive it, and the assumption is that "Well, 2 channel is good enough-they're Macs, and that'll work okay.
Dr. Lex (www.dr-lex.be ) has made an enabler which allows these basic boxes to work on Mac. as long as they have a certain chip in them, then this will work. these boxes might work over usb....
hopefully this helps....if not post here again and someone may help you. If you can, maybe a model number of your logitech speakers might help us-not all 5.1 speakers are the same....
JB