Thanks for the ST community developed Echo Speaks, it allows great integration between Hubitat/SmartThings platform and the Amazon Echo devices, and I have seen many users using the Album Art feature and rules to create their own unique “jukebox dashboard” with their echo devices like mentioned in the post below.
So I’d to share a trick how to create an automation rule to play music to multiple echo devices via “Multi-Room Music” group here. 
Echo devices sometimes have issues with Multi Room Music groups (MRM). See the post below and the linked GitHub issue for more details.
See steps below:
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Create a “Multi-Room Music” group in Alexa app
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Open the Alexa app on your mobile device, and go to “Devices” tab
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Tap the “+” button at top-right corner and select “Set Up Multi- Room Music”
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Give this group a name and select the desired echo devices ( I named this group “All speakers” in this example.)
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After the group is created, you should be able to see this new group listed under “SPEAKER GROUPS” section as a “Multi-Room Music” item.
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Create a rule to play your favorite music to this speaker group
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In the Flow section, add an action and select either of your echo speaks devices. It doesn’t matter which of the echo was selected as long as it is one of the echos in the Multi-Room Music created above.
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Select “searchSpotify” as the command (or you can select different one, such as “searchAmazonMusic” or “searchAppleMusic”, depending on what your music source is)
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Tap the “Advanced” option at top-right corner and removed the two “Number” fields in the parameter list since we don’t need to adjust the volume and sleep time here. We only need to keep the “String” field here for the command parameter.
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In the String parameter field, specify the music station, play list or song name and append " on All Speakers" text after the station/play list name. Ex: Beatles is my playlist name and All Speakers is the name of Multi-Room Music (Speaker Group) we created from the first step.
According to the Echo Speaks Document, the music search command supports the audio group to be in the search parameter.
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Depends on if you want to trigger this rule by an event or manual tap from the dashboard, you may or may not need an trigger in this rule. Once done editing the rule, go ahead to save the rule and give it a try if it is able to play your favorite music on all the devices in speaker group you defined.
Tips: if the String parameter mentioned in Step 3 doesn’t work for you. Try different voice commands to play your desired music on the speaker group. Find the working one and use that voice command content as the command parameter in Step 3.
Now you are ready to play the party music through the entire house then.
