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Go to the phone's "Settings" - "Sound", find the "Media" option, and turn up the "Media" volume. The same goes for car navigation systems.

First enter the Bluetooth settings interface of the mobile phone. There are two options: mobile phone audio and media audio. After connecting to the car Bluetooth, these two options are turned on by default. We need to turn off the media audio. And retain the mobile phone audio, so as to ensure that the three functions of Bluetooth calls, car playing music (or radio) and mobile phone playing navigation sounds do not interfere with each other.