When you use your car’s built-in speakerphone feature, the speakers in the car door reverberate, sending the sounds outside of the car. Maybe don’t do this while your car is parked. Everyone in the neighborhood or store parking lot can hear your very intimate or business-related conversation.
Aye, this is one of those weird things that I’ve noticed. The conversations somehow broadcast outside the vehicle much louder than music. Something about how voice calls are handled by the sound system is different, whether it’s volume boosting or something else.