Audio problem

Hi there,new member here saying hello to everyone and also looking for some advise. I have just bought a freelander td4 serengeti 2003 and I have changed the factory cd player to a sony mex bt2600 bluetooth unit for handsfree mobile phone use, but the sound is coming through all speakers when radio or cd is playing but only a very low and "tinny" sound when a call comes through, can anyone help.I dont know if I need any other cable adaptor or not.