As you are already aware, the chime is provided via the audio system. However, it is the body control module that signals the audio system of an issue.
Since the radio is aftermarket and the system is working but softly, I suspect it deals with the radio itself.
I was able to find one schematic for the audible warning system, which I attached below. Take a look through it and let me know if it is helpful or if you have questions.
See pic below.
Monday, October 17th, 2022 AT 6:49 PM