I connected the Apple TV (4 gen) to the airport express via AirPlay and the airport express is connected by a toslink to a dac.
The problem is that the movies's audio is seen by the dac as Dolby Pro Logic (surround) but not like Dolby Digital 5.1.
Why? Does AirPlay do not support 5.1 DD?
Correct. Airplay works best for web-based video, video from phone apps, or mirroring. Actual movies or TV shows on your iTunes can be played through the apple TV using home sharing. Purchased videos in iCloud should be accessible without even that. So, for content with surround audio, airplay is not a good option.
Airplay (on the device that is sending) transcodes the audio into Apple Lossless format, and is only stereo.
AirPlay Dolby 5.1
