No sound over spdif

When you have selected to use audio via optical out (S/PDIF), and have also connected your receiver to your XBMC system using an HDMI cable, please note that the digital sound may be carried only via the optical cable, and that the HDMI cable may 'transmit' silence to your receiver.

If you switch to S/PDIF and get no menu sounds, nor any sound is played via streaming, nor via any music or video source, check your receiver settings to see if you can change the input mode from 'Automatic' or 'HDMI' to 'Digital' to let it recognise the digital signal via S/PDIF.