First, verify the XLR to mixer and USB from mixer connections are secure. Be sure to check that both the input for the microphone is at a reasonable level and that the main line output for the mixer is at a reasonable volume.
If after the first step you still do not hear audio, next verify that the mixer input is enabled by navigating to the audio settings from the system settings icon. The name will display under the Inputs section.
Another way to locate point of failure is to attach headphones to the 802 mixer. If audio is heard, the microphone is working. If no audio is heard, there may be a faulty mixer, faulty USB cable, or faulty USB port. Attempt to swap the USB cable or connect to a different USB port on device.