CarPlay and Android Auto work through screen mirroring. In this mode, the audio output (music, calls, and navigation prompts) is generated and controlled by your phone, not the display.
Because of this, volume control remains with the source device—this is an inherent limitation of screen mirroring systems.
How to adjust volume instead
Please adjust volume using one of the following:
Your phone’s volume controls
Your Bluetooth headset / intercom volume controls