I finally decided how to use the free space in the lower right corner of the gui: I added an "Audio Input" panel.

Now you can use the sidechain as a modulation source and you may route it through the synth.

It is also possible, to use 2 different signals, e.g. a modulation signal on the left channel and an audio source on the right channel.

AudioVolume and AudioPanning are added to modulation destinations.

I think this is a good solution ... and also sounds very interesting^^