I'm trying to use the optical audio output of my motherboard to send the sound to my AV Receiver.
I only see the front channels received by my AV.
Indeed rear channels don't seem to be activated.
How can I activate the rear channels ?
You can't. S/pdif support PCM, AC3 and DTS (if your decoder support them) PCM only support stereo up to 24bit/192khz
AC3 and DTS support surround. But they are patented technologies and not open-source friendly. As far as I know, ther is no way to get Ubuntu to transcode on the fly to AC3 or DTS to feed S/PDIF