maybe you can share some details about your application.
I don't think you'll be able to build a digital decoder as cheaply as Creative Labs, which is packaged with every sound card they sell these days, not to mention the nock-off's and clones that are even less expensive. do you want to decode a s/pdif signal, that is not coming from a computer? for example you have a stand alone dvd player with only digital outputs, and you want to connect analog speakers?
I found a simple circuit here that decodes the main left and right channels:
**broken link removed**
perhaps that can be used as a building block to extract all six (or more) channels.
I think the jist of what you need is something to demultiplex (not the correct term) the individual channels from the single PCM bitstream (most of the time s/pdif is in pcm I think), and pass the individual bitstreams onto a multichannel DAC, and then from the DAC to a preamp, and then you've got analog
check out Cirrus Logic:
www.cirrus.com
they have the chips that do just that... specifically the CS42516 ... it includes an 8 channel SPDIF receiver / decoder and six DACs - The reasoning here (i believe?) is your source could have a prologic stereo audio stream as channels 1 and 2, and have channels 3 through 8 as the dolby digital surround sound bitstream, the chip also has digital volume control and two onboard ADCs. Looks like the chip is controlled via I2C, so any suitable microcontroller should handle it. There is also what appears to be a digital output of some sorts. The chip only comes as a 64 pin flatpack - so you'll need a prototyping adapter or some PCB fab skills to work with it.