The volume suddenly drops sometimes.

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. Use the [OFF, ON, TALK OVER] selector switch to turn the feature on/off.

For details, see the Operating Instructions > [Microphone] > [MIC section].
You can read the Operating Instructions [here].