To add Raspberry Pi Volume Central, we need to add some components (Automation, Shell Command, Group, Script, Input Sider) and add some files to your .homeassistant directory.
Go to: Home Assistant Raspberry Pi Volume Central on my site for the fall instractions. 
Hi Laith
What device’s volume is being controlled? I want to connect my bluetooth speaker to my Pi3 but don’t know where to begin 
Thank you
Hi starman.
This will control the raspberry pi volume not you Bluetooth speaker. This means that volume level being sent to your Bluetooth speaker is been adjusted. If you want take full advantage off your Bluetooth speaker volume you should keep it at 100% on your Bluetooth speaker and then adjusted from home assistant.