The last 3 seconds of each phase and the final phase as well may come along with some sounds. 4 default-sounds are built-in.
If the user doesn't like those sounds, he can replace them by installing one or all files "sound1.mp3" ... "sound4.mp3" in the smartphone file system, in the directory "Tabata/sound". The full pathname depends upon the device and the version of the operating system. Example: "/storage/emulated/0/Tabata/sound/sound1.mp3".