Talkover might be turned on.

When it’s on, this feature automatically reduces the volume when the microphone picks up the sound from the speakers. Try to use the microphone as far away from the speakers as possible.

This issue may be caused by static electricity etc. If you don't need to use Talkover, turn it off.

For details, refer to the Operating Instructions > "Advanced use" > "Using a microphone".

You can read the Operating Instructions [here].