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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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Volumio can handle multiple devices in order to playback simultaneously the same music source up to 6 rooms (devices) on the same local network.
First:
Go to the SETTINGS menu
Go to SOURCES menu
Make sure the Multiroom Playback is enabled
Next:
On the HOME PAGE, click the OUTPUT ZONES icon
Add your devices to the main player to create a group
Every room (device) will have its own volume control
You can create more than 1 group with more than 1 audio source.
Every single group can play different music sources.