Panasonic SA-HT870 Help needed?

Hi i just recently bought this unit off ebay and i have had problems ever since , first i noticed that the cable that connects the dvd to the sub has been damaged (Pulled out) and silver cable is exposed but all the pins are fine . I recently neatened this up but when playing music on medium volume the system goes very quiet , the bass stays ok but the voices go quieter . I know nothing about this unit but can follow instructions easily so if anyone knows what could be causing this please comment and that would help alot .