Optical Audio On PS3 Question

[ellldiablo]

I connected optical audio cable to a bose 321 system and after changing the audio output from hdmi to optical, i did not get any audio. when changed back to hdmi i had audio. did i miss something? is there another setting i did not change in order to get the audio on the 321 system which is only 2.1 if you where unsure. for the life of me i could not figure out what went wrong and the bose system is fully working, checked it with a dvd player with optical audio out.