The owners manual indicates the way you make changes to how you integrate internal and external sources is by sequencing performances in live sets. See sections on scenes, live sets and master keyboard.

The scenes seem to be heavily focused on managing changes "simultaneously and drastically" to motion sequences, arpeggios, muting, etc within a performance. As a master keyboard, seems managing external and internal sound sources are set as a performance snapshot using the UTILITY mode; of course, by muting a part(s) and "snapping the scene" in a performance, one can turn sources on and off within a performance.

Is it possible to use scenes to control external sources?

Example:
Can I programm that the Montage will send out volume (cc7) = 100 on channel 1 and volume (cc7) = 50 on channel 2, when I push scene-button 1?

No. The Scene is used to memorize internal settings, it does not send cc messages, that is not how it memorizes settings, at all.

I mean, can I use a single scene-button to make the right mix, external and internal?

The Scene button will affect the Montage settings, the Master Keyboard function can store the volume settings for your external devices. But there is one of these per Performance.

Thanks Mike, thanks Phil.

So the answer to quickly change the mix of internal and external sources is just use several performances.

(Or use a scene-button or use the Superknob for intern and turn some knobs for extern.)

Anway, I think I can live with it 🙂