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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
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balena-sound
Build a single or multi-room streamer for an existing audio device using a Raspberry Pi! Supports Bluetooth, Airplay and Spotify Connect
Instead of splitting audio from the jack output you can create a Little network with a router and have more rpi streaming music over It using snapcastsnapcast .
You can create have a master and a slave, and control audio flow from remote over the network with an android app or with a webgui .
Have a look at the Balena Sound project on Github here and other possible options would be Volumino or RuneAudio - they are not up to professional multi-room sound systems but may give you some of the functionality you want.