AirPlay-device in Audio-Midi App

Ok, just to be clear. One of the networking protocols Apple uses for AirPlay to work is Bonjour. In turn, Bonjour is based on mDNS. Regardless, mDNS is not normally routable to cross network subnets/VLANs. To be able to get mDNS to travel from one segment to another, it would require a mDNS reflector. Most consumer-grade networking hardware do not offer this feature.

If you are using 4 audio sources, each on their own WLAN segment, then any AirPlay devices on those respective WLANs should be able to get those AirPlay streams. They just can't get them across WLANs (again. only if each of these WLANs are on their own separate subnet.)

jotfisch wrote:

Is it possible sending several different stereo-files at the same time in to more than one WLAN networks?

Streaming relies on AirPlay, which is not routable without a network that can support a mDNS "reflector." So attempting to use AirPlay over two separate WLANs (I'm assuming that is what you mean) on different subnets will not work. In addition, AirPlay can only stream one source at a time, but can do so to multiple destination AirPlay-enabled devices. Streaming multiple stereo-files can only be done in series.

Is it also possible, to connect to more than only one AirPlay-device at the same time and combine them to one aggregate device?

With AirPlay 2 you can stream the same audio source to multiple devices ... which is not the same as them being an "aggregate" device, but should work in a similar fashion.