My guess is that it is a train airhorn they use as a signal horn.
I do not think it is the siren on the first vehicle.
The first vehicle is from Friwillige Feuerwehr Sayda and the second vehicle from Friwillige Feuerwehr Sehma.
Both uses of them are fire trucks made by ЗиЛ (ZIL) Zavod Imeni Likhacheva.
The second vehicle has the siren I mentioned earlier in the grill.
It is a four piped compressor airhorn siren with the common German hi/lo sound.
I guess that the first vehicle has something similar.
I can not see any horns, electronic siren speaker or motor siren on the first one so I assume it has an under hood mounted airhorn siren.
Why not contact them on e-mail address:
feuerwehrsayda@aol.com and ask them of what it is?
Here are some photos of the two vehicles I took from their web sites.
Michael