No Volume on 1992 Volvo 740. Help?

I recently installed a battery in my 1992 Volvo 740, with the factory audio set-up. I put the radio code in and that all works fine. The only problem is when the volume is all of the way up I can only hear it faintly. What could be causing this problem? If its a fuse or an amp of some type would you be able to tell me where I would find it? I aprecciate any support. Thanks