MindBell has its own volume settings, therefore the bell volume is not influenced by your Android volume settings.
In the active-times-and-rhythm section of the settings you can change the volume of the mindfulness bell.
To modify the volume of the meditation bells take a look in the meditation section of the settings.
From version 3.2.5 on you can choose which audio channel to use for sound output and whether to use your phone’s volume control instead of MindBell’s volume settings:
Please check the sound output section of the settings.