When the USB connection is in use, sound doesn’t output from the mobile device. Why not?

Sound doesn’t output from the mobile device when the USB connection is in use.
Connect a powered speaker, powered amplifier, etc. to the [MASTER] output terminal on the unit.

Note: 
When connecting an iPhone or iPad with a USB Type-C port, you can easily enjoy DJing with low latency without external speakers by the following methods.
Enable [Output audio from the iPhone speaker only] or [Output audio from the iPad speaker only] in [Settings] > [Audio] in rekordbox for iOS. 
If enabled, audio output and some operations will be restricted as follows.
・Listening with headphones connected to the unit is not possible.
・The audio output can't be adjusted by turning the [MASTER LEVEL] knob on the unit.
・No master audio is output from the unit.
・Audio from the microphone connected to the unit won't be output from the mobile device.