yes... Same problem but more details:
The airport express (2 different ones) are at revision level 7.8.1 newest version.
When 11.3 came out, on my M1 MBP, streaming to the airport express stopped working. the status shows Selected (in any app and control panel and top menu bar) and sound stops on internal speakers.... but nothing from the attached airport express.
It still plays audio on the Apple TV device... just not the Airport express.
I happen to have a product by Rogue Amoeba called Airplay installed... it still works to the Airport express, Apple TV and other Airport Express (all at the same time).
I hoped that the new 11.3.1 update would fix the problem but it does not.
In addition, when connecting to the Airport express, It always asks for the device password even though the password is stored in the keychain.
My conclusion: There is a bug in 11.3 that killed airport express. This is not good. I hope that it is on the bug list to be fixed soon.
thank you