Jameswei is right. Also, there may be no problem at all. If the image of whatever you are listening to is not exactly center, then the volume/loudness will be different for each channel. For example, if the trumpet you are listening too is to the right of center, then it should louder in the right channel.
Its also possible that the volume pots need to be cleaned.
Its also possible that the volume pots need to be cleaned.