Check here first for the sound you want https://github.com/PocketMine/PocketMine-MP/tree/master/src/pocketmine/level/sound and I used google to find this. --> https://forums.pocketmine.net/threads/what-is-the-function-to-play-a-sound-to-the-player.8944/
Thank for answering, I checked the sound phps. But, I cloudn't find the sound. And I know how to play with addSound() Can play the Level up sound?