Make sure your audio source is properly connected to the corresponding inputs of your audio hardware. Make sure that the physical inputs of your audio hardware are defined as input for the meter in which you want to visualize the audio signal. This is done in the "Meter Configuration" in the "Options" menu. If there is still no visualization, check whether the meter is turned on, by right-clicking into the meter and making sure that "On" is selected. Make sure the correct samplerate of the input signal is selected in the Control Bar