connecting speaker

I don't think you need a sound card - all you need is a mini stereo to dual RCA adapter and some RCA cables -

http://sewelldirect.com/Gefen-Mini-Stereo-to-RCA-Adapter.asp


If the Z5300 has digital optical input, it would be better to use the Belkin PureAV Digital Optical Audio Cable with Mini-Toslink Adapter found at the Apple Store (apple.com/store).

I think you are right - I can run my laptop through our Yamaha sound system on our home entertainment center. I just use a mini to RCA adaptor and plug the mini end into the laptop and the RCA into the auxillary input on the Stereo receiver. All I have to do is switch the system to auxillary, and the sound from the laptop goes through the Yamaha system - sounds great! Nice way to use our iTunes music for parties.

Eventually, I would like to get an Airport Express to be able to do it wirelessly, but for now the adaptor is fine.

Scott

Since the speaker system doesn't seem to have a digital input, you'll only be able to use the left and right (stereo) channels with your MacBook.

What you need is an external USB or FireWire device that outputs at least 5.1 analog channels, for example the Miglia HarmonyAudio:
http://www.miglia.com/products/audio/harmonyaudio/index.html

However, you cannot use Apple DVD Player to play Dolby Digital 5.1 tracks, because it passes on 5.1 audio only to the digital Toslink output of your MacBook. It cannot decode the sound into 6 separate channels. You'll be forced to use something else, such as VLC:
http://www.videolan.org/vlc/

The easiest solution is to just get a different speaker system that has an optical digital (Toslink) input.