If you are using Android your phone can manipulate the volume using the volume buttons.
But if you have iOS, you phone can't do that, so you might need to use external buttons to modify the volume. I this project you will learn:
1. How to configure the raspberry pi device tree to access the hifiberry amp2 as a sound card.
2. How to configure the raspberry pi bluetooth in order to use it as receiver of audio stream over bluetooth.
3. How to use alsaaudio mixer in python
Note: I am using the raspberry pi zero w, the hifiberry amp2 two speakers of 4 ohms/15W and a power supply of 18V.
See the repository here: link