The plug of the external microphone or speaker is dirty. -> Wipe the connection terminal of the microphone or speaker.
The external microphone or speaker is not connected properly. -> Correctly connect the external microphone or speaker.
The audio from your PC speaker is being caught by your PC microphone, and then the camera is transmitting the audio as the noise. -> Check [mute] for the microphone column on the volume control screen. Then the PC will stop transmitting audio to the camera.