What happens before that? (Maybe OS notification shows / game is being minimized)
Does it change the value on volume slider in the game?
If volume stays unchanged in the game but it sounds louder, I'm afraid it may be caused by some bug in your system libs

no, there was nothing like that. it's just that in the last few weeks the music in the game has been getting louder. this does not work if music and sounds are disabled in the game. perhaps the volume of the music when turned on is lower than the set value and then suddenly returns to normal. it also doesn't happen with other apps on the phone and I recently confirmed it happens on headphones as well.

Additional info. This started happening to me a few weeks back. About 5 minutes gameplay and the volume maxes out. When pausing, the music volume snaps back to where the slider is set, I assume. Bug then happens again after some time. Google Pixel 3a