There are 2 ways to change the volume of a sound:

1. Tap Mixer in the Bottom bar to display all sounds currently playing. From there, you can adjust individual sound volumes or remove them if you tap the delete icon.
2. Tap a sound. A volume slider will then appear on top of the screen and you can adjust the sound volume. You can also change it if you tap and hold its icon: this will bring up the volume slider if you already selected the sound.

You can reset the sounds volume to default in More/Settings. Tap Sounds volume and they will be set to their default intensity.