Airport Express - no sound

I have the following setup:

Zone 1: La Rosita - streaming via HQ Player - works perfectly fine, excellent quality
Zone 2: Airport Express + Harman Kardon Aura via Airplay. Both devices are correctly recognized by Roon. And actually Roon says that both receive and play the music well. But unfortunately there is only sound on the Aura - not on the second Airport Express.

This is even more strange as it worked already - so there was sound on the Airport Express.

I did already do the following:

  • restart Roon core
  • checked Network
  • reset the Airport Express
  • checked physical wiring

I am really running out of ideas…
Thanks for your support.

Sorry to be so basic and looks like this is an old thread.
Did you get it figured out?

If not my two guesses would be check individual Express volume and after that try connecting to it via iTunes.