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You are on Volumio 3 and have a Premium subscription
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Click the menu icon and go to Sources
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Enable Multiroom Playback
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On the main page, click the output zones icon
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Volumio can handle multiple devices in order to playback simultaneously the same music source up to 6 rooms (devices) on the same local network.
1:
Go to the SETTINGS menu
Go to SOURCES menu
Make sure the Multiroom Playback is enabled
2:
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.
3:
Control the volume of each device from SWITCH DEVICE section
From there you can remove devices from the group