Hi Andrea, plug the audio source to one of the AUDIO IN Sockets (3,5 Minijack) in the MASTER module; then you can patch it from the pin socket on top right to further processing with filter etc. Please note that the internal audio level in the AE is 2...3V p-p , so perhaps he SIGNALAMP module might help. If anything remains unclear please let us know!
Hm...I see the possible need here for coming up with an envelope follower, something which can track the level contour as well as provide trigger/gate with an adjustable comparator. Combine that with the Master's inputs, and then you have all of the Doepfer A-119 functionality...plus perhaps a little bit extra if both a trigger on comparator-crossing is there in addition to just a gate.