Stream any audio from Android phone to multi-room speakers?

Is it possible to stream any audio from an Android phone to a multi-room speaker setup?

I would like to invest in some multi-room speakers, but I do not use any streaming services. My media files, both audio and video, are scattered around locally on my hard drive and phone. I do not have a central media library. I watch a lot of videos on my phone’s YouTube app.

IKEA’s SYMFONISK speakers seem like a good set of entry-level devices to set up a multi-room audio system. However, I’m unsure about how to actually stream music to them. To my research I can only use streaming services or a media library as input.

I also own a Bluetooth headset that I can connect to my phone and it will play any auto, whether it is music, video or YouTube content. But to my current understanding the Sonos App is required for IKEA’s speakers and I can only stream audio that I explicitly selected within the app. The speakers do not simply ‘forward’ all audio of my phone, correct?

Long story short: Is there any solution for a cave man like me, who does not rely on streaming services? My phone would be most important for me, so don’t bother about my Linux laptop too much.

To my current understanding, I would be better of with a Bluetooth speaker instead of Wifi speakers. But Bluetooth speakers cannot be used as a multi-room setup and connectivity is always a big issue.