Are there any alternatives (for multi-speaker synced audio) since the Chromecast Audio was discontinued?

Have a look at these: bluesound, audiopro, bang-olufsen, denon.

Depending on which software is in use for streaming operations, you could also use ‘standard’ WiFi-enabled speakers like something from KEF (LS50) or dynaudio.

Just from top of my head. Maybe someone else has a comprehensive liste at hand.

Soundboks 4 sync wirelessly with each other. They’re also battery driven and absurdly loud, so perfect for bringing outdoors or a dance party somewhere.

They just had a 30% sale for the holidays though, so might not be worth getting them at full price

I’m still using chromecast audio they work great!