the volume is low when the caps aren't charged yet. It should restore full volume when they become fully charged. So something is off here. Like stated, the volume settings should be checked. When you state ten minutes, do you mean at more than 12 volts for that time? Like 18 volts? If low voltage, maybe it's not enough getting to those caps.
A cap test could be done by letting the engine idle and kill the power. See how long it plays shut down sounds for.
I'm guessing you felt these engines used to be louder, right? It may need to be looked at. Even the speaker should be inspected.
