A dumb question - what audio is on Bluray DVD's ? (did not have time to research it)
From wikipedia (http://en.wikipedia.org/wiki/Blu-ray_Disc#Audio
For audio, BD-ROM players are required to support Dolby Digital (AC-3), DTS, and linear PCM. Players may optionally support Dolby Digital Plus and DTS-HD High Resolution Audio as well as lossless formats Dolby TrueHD and DTS-HD Master Audio. BD-ROM titles must use one of the mandatory schemes for the primary soundtrack. A secondary audiotrack, if present, may use any of the mandatory or optional codecs.
So by default all BD must use a format which is supported over S/PDIF (ac3, dts, 2ch-pcm).