Is the microphone talk over function on, and is a microphone that is on and connected to this unit placed near a speaker?
When the microphone picks up the speaker sound, the talk over function is activated and makes the output sound smaller unexpectedly. Turn off the talk over function or the microphone, or use the microphone away from speakers.
The talk over function may be accidentally activated due to the influence of static electricity, etc. If you do not want to use the talk over function, turn it off.