Volumio can handle multiple devices in order to playback simultaneously the same music source up to 6 rooms (devices) on the same local network.
Go to the SETTINGS menu
Go to SOURCES menu
Make sure the Multiroom Playback is enabled
Click on the GROUPING icon, inside the OUTPUT MENU
Add your slave devices to the master player device in order to create a group (Green GROUPING icon will appear only on MASTER DEVICES)
If you want to create a different multiroom group go to SWITCH DEVICE section in order to change the master player device and repeat the same flow
You can create more than 1 group with more than 1 audio source.
Every single group can play different music sources.
Control the volume of each device from SWITCH DEVICE section
From there you can remove devices from the group