The [VOLUME] knob on the rear panel of the SPD-SX PRO adjusts the gain of the audio signal being input to the AUDIO IN (LINE jack). When you raise the gain with the [VOLUME] knob, noise from the input audio is also amplified, and you may notice a low hissing noise even when no sound is being made. If this happens, try using the noise suppressor.
Refer to “Configuring the Input/Output Jack Settings (AUDIO SETUP) – AUDIO IN” for how to configure the noise suppressor.