About UI Sound Kit 2
All mobile devices have built-in processors at the audio output port. It pulls out all the sounds, however, and the speakers of the devices are unable to reproduce the sounds at high volumes with high quality. Perhaps you will be surprised at how awfully bad and wrong something sounds that you quite liked listening to with your computer."
That's the truth - loud audio files are sounds very distorted on mobile speakers. It's recommended to use quiet sounds for UI. And sometimes it's ok to use loud sounds for notifications, but you should be careful and pick the right sound.