I recently upgraded to XBMC Frodo 12.2 and lost my sound over SPDIF. The option for SPDIF wasn’t available in the options.
After some searching around, I found the following forum post;
1. Set speaker config to 2.0 or 2.1 in XBMC.
2. Set speaker config to 2.0 in osx audio-midi-setup
3. Activate AC3 and DTS options in XBMC
4. Disable LPCM otption in XBMC