It does sound very like there is only one channel playing through both speakers.
Try listening very carefully to see if there is any separation between the channels. If all the sound is 'in the middle' you are effectively operating in mono.
If the speakers are plugged in correctly you may have a more fundamental problem and the unit may require professional attention.
I had a similar problem with a Beocenter 2000 where there was only one amplifier channel working and a previous owner had masked this by soldering internal wires acrioss the two sets of speaker outputs so every speaker output played the left channel!
Simon