Assuming that you have proved whether it`s the speaker or radio?
Assuming it`s the radio, then a common fault is the output transistor "blowing" on one channel (ie..going short circuit and also destroying a resistor or two.)
Can be tested using a meter.
Can usually be repaired by D-I-Y or at a repair shop.
Ask here for advice re price etc.
http://www.davidsonelectronics.co.uk
Angus
June 2008