The PS3 doesn't have a multichannel analog output - it has stereo analog and optical digital only. Setting it to PCM would just cause it to downmix Dolby Digital ad DTS to stereo. Also, the optical output and bitstream is the preferred output for Dolby Digital and DTS - it's only the Dolby Digital Plus and TrueHD (and maybe eventually DTS-HD Master Audio) that can't be carried by the optical.