Door chime is quiet?

The door chime is operational but barely audible. The factory radio was replaced with an aftermarket one years ago and I have had no problem with the chime until recently. The chime is working properly with the key in the ignition and door open, headlights on door closed but you can barely hear it. It was working fine up until a few months ago. I am an electrician and can troubleshoot but there are no schematics to the circuit. I have a factory repair manual and it does not address the chime circuits and its associated switches or locations.