I have a sneaking suspicion that your right back speaker is hooked into the front right speaker on your reciever. The tv signal might only be two channels of sound, right and left; your reciever might not send any of that sound to your rear speakers. And if your reciever thinks that your front right is actually a rear speaker, than voici le problem. The DVD and CDs might send sound to all of your speakers, making the mixup unnoticeable at first.
To test my theory, try to find the feature on your reciever that allows you to test each speaker. The reciever should concentrate sound into each speaker one at a time. That way you can tell if the reciever is hooked up right.
Oh yeah, and if I'm right, be sure to poke fun at your friend next time you see him.
