To understand how SPDIF work, please refer following link for detail.
http://www.spdifworld.com/hardware-specifications.html
To connect the audio output through SPFID, please follow the instructions.
1. Connect a SPDIF bracket cable (coaxial+optical mixed) into MB SPDIF OUT connector.
This bracket must with "signal transfer controller", or cable need purchase from Biostar dealer.
SPDIF%20OUT.JPG)
2. Check the SPDIF Bracket optical LED must be light-up.
If not, try to install the audio driver again from driver CD.
Bracket_Optical%20LED.JPG)
3. Speaker connect to H/W Digital Decoder.
Decoder_connect%20speaker.JPG)
4. Connect SPFID with coaxial cable.
Digital decorder select coaxial function.
Decorder_select%20Coaxial.JPG)
Use a coaxial cable from bracket coaxial connector to digital decoder coaxial connector.

5. Connect SPDIF with Optical cable.
Digital decorder select to Optical function
Decorder_select%20Optical.JPG)
Use optical cable from bracket to digital decoder's optical connector

If there is no sound output, try to double check audio driver's utility is select for SPDIF/Digital output.