Try these solutions:
1. Ensure that the USB Audio source has been selected by selecting the Bluetooth/USB Audio button on the front panel.
USB Audio source is shown by the symbol being lit orange.
2. Ensure that your PC/Mac is connected to the USB Audio input with a USB A to B cable
3. Make sure that the correct USB driver is downloaded
Our range of drivers can be downloaded from our Driver Updates page
4. Ensure that the CXA is set to USB Audio Class 2 in the Setup Menu