stellgp :
Thank you for your reply! This Sony home theater system does not have the 5-channel stereo mode. But, I tried the HDMI mode (ON) on Settings (not the HDMI + Sound Mode - rear speakers get disabled if you choose that), and then the sound modes worked differently: with ProLogic the rear speakers worked but in very low volume, with C:6 Cinema and C:6 Music they work fine and clear, and with Sound Mode On only the center speaker works (even the front L+R are disabled in this mode). Boy, it took a while (lol).
Usually the music will only be in stereo anyway, so the system is working correctly in that sense. Also, dont forget that when you have a sound track on a film or a game etc that is in 'surround sound' the rear speakers will be much quieter than the front as this is also normal. The rear speakers in movies almost never produce the same amount of volume as the rest. You could look in the menu and see if you can adjust the gain for the rear's (+/- db). Or on some A/V units, you can adjust the distance that the speakers are from where you sit, which will make them louder or quieter.
Good on you for persevering with this though, its the best way to learn your way around a system!
Hope this will help you more.
K.