syno-plex-update VS AirConnect-Synology

Note; Plex in Synology Package Center isn’t usually up to date, so using a script like this one to automatically update would be your best option (especially for security updates, etc.)

There are a few dockers that seem to be able to do what you want. Some can stream local audio to airplay devices what might work with a bit of scripting for your white noise and others might work as a Chromecast to Airplay bridge https://github.com/philippe44/AirConnect https://github.com/1activegeek/docker-airconnect

AirConnect - "Use these applications to add AirPlay capabilities to Chromecast and UPnP (like Sonos) players, to make them appear as AirPlay devices. AirConnect can run on any machine that has access to your local network (Windows, MacOS, Linux -x86, x64 and ARM, Solaris and FreeBSD). It does not need to be on your main computer. (For example, a Raspberry Pi works well). It will detect UPnP/Sonos/Chromecast players, create as many virtual AirPlay devices as needed, and act as a bridge/proxy between AirPlay clients (iPhone, iPad, iTunes, MacOS, AirFoil ...) and the real UPnP/Sonos/Chromecast players."

Not sure with HOOBS or HomeBridge. But there are projects like AirConnect which turn Sonos or Google Cast/Homes into AirPlay v1 targets.

I know there are several tools to stream music to chromecasts via airplay protocol (AirConnect is one of them, CastBridge another one) I'm looking for the other way around; Using the Chromecast protocol to send audio to airplay devices?

I've installed AirConnect on my Pi and it's working great so far. I can even pause and stop playback from my Home Mini. (Skipping doesn't seem to work). Even groups work. (For those interested, it should also support other devices like older Sonos speakers).

I see you mentioning that you should be ok with all speakers having Airplay (2) support. You may patchwork something useful together on the RPi with Homebridge (for - clunky - HomeKit support) and this (Airplay (1) support for each speaker): https://github.com/philippe44/AirConnect

I am using AirConnect for a few years now and i am also maintaining the AirConnect package for Synology NAS and Router devices on github: AirConnect-Synology If you have a synology device, you only need to download the specific package and install it on your synology device.