The sound is heard from the mobile device when I select USB as the source.

The sound output may come out from the digital media player or mobile device if playback has been started on the application before connecting it to the USB port.

Close the application and try connecting the device again. If this does not resolve the issue, refer to the operating instructions of the digital media player or mobile device.

NOTE: Updating the firmware of the digital media player or mobile device may improve functionality.