The volume suddenly drops sometimes. Why?

Talkover might be turned on. (The MIC ON button is blinking)

When [ON] is selected, the Talkover 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.

Talkover OFF: the MIC ON button lights up (press the ON button without pressing [SHIFT] button)
Talkover ON: the MIC ON button blinks (hold down [SHIFT] button and press the ON button)

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