Latest version for Airport Express 802.11n (first generation)
There are two models of Airport Express 802.11n, a "first generation" (brick shape) and "second generation" (puck shape). Since the release of firmware 7.8, the firmware versions table at Update the firmware on your AirPort base station - Apple Support doesn't distinguish these two models.
My Airport Express 802.11n first generation doesn't see any firmware updates after 7.6.9, so I assume I can't use Airplay 2. If that's correct, this page should be updated.