If you use a TomTom Car Kit or other hands-free car kit you can hear warnings from the app on the car kit using your phone's Bluetooth connection. Follow the instructions that came with your car kit to connect your phone to your car kit.
Some car kits put the Bluetooth connection in sleep mode if a signal is not received from your phone. This can cause delays when the connection is required. To keep the Bluetooth connection always active, switch on TomTom Car Kit in the Settings menu.