TomTom GO GPS - Manual (Page 20)

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Your device may require you to enter a password on your GO before a
Bluetooth® connection can be established. See the documentation for your
Bluetooth® Hi-Fi device for more information.
Once connected, sound will be played through the external Bluetooth® Hi-Fi
device.
If your GO cannot establish a Bluetooth® connection then all sound will be
played through the internal speaker or through the audio cable if it is
connected to an external audio device with speakers.