The MODX (in default setup) will output cc7 when the “Slider Function” is set to “Part Control“ — they are Part Volumes, 1-16. This allows each Part 1-16 to generate a Control Change message (cc7) that when captured and played back to the MODX will control the Part Volume. The messages generated by the MODX controllers in its default setup are designed to be captured and played back to the MODX.

When you select an individual Part, for example, the role of the Sliders can be changed to control individual Oscillator output levels of the currently selected Part. These are not handled by cc messages but are Parameter Change (Sysex) messages, exclusive to the MODX synth engine.

Therefore, the Sliders only send cc7 when you have selected a condition that sets them as Part Volume.
Press [SHIFT] + [HOME] to view the “Motion Control” > “Overview” screen
When “Slider Function” = Part Control, each Part has a Volume Slider. Use the front panel buttons to toggle [1-4/5-8] and [SHIFT] + [1-4/5-8] to reach all sixteen Parts in sets of four.
When “Slider Function” = Element/Operator, the selected Part has a slider for each Oscillator; Elements 1-8 if AWM2, and Operators 1-8 if FM-X.

The prime function of these Sliders in the default mode, is to take care of the MODX.

If your goal is to control external devices, intelligently, it is highly recommended that you use the Zone Master (Master Keyboard) function. This allows you greater control that can be targeting a specific external device, particularly when communicating to external hardware via standard 5-pin MIDI.

The Zone Master function can be configured for use via USB-MIDI — where a MODX Part is created, then is re-routed to an external device via MIDI. You literally substitute the external device for one Part slot of the Performance. But because in this configuration both internal and external Parts can be under your control, the role of the MODX Sliders remain CC7 Part Volume when “Part Control” is selected and Sysex when an internal oscillator is the target.

The MONTAGE (only), additionally, has a REMOTE layer that communicates exclusively via USB Port 2 with external DAW software (Cubase, Logic Pro, Pro Tools, Ableton) and can run these DAWs remotely, including reassigning the entire front panel to control DAW plugin synths and effects. A REMOTE layer re-tasks the front panel as a control surface - this is the ideal situation when controlling an external software or software based device.

Summary:
The default messages the MODX generates are designed to address the MODX itself.
The Zone Master function allows you to intelligently design a Part/Zone slot to address each internal sound and/or each external device. For more on this please see this article: Zone Master and MIDI I/O Modes
A DAW Remote function is the ideal way to control external software based devices as it reassigns all front panel controls to address the external device... on its own discreet MIDI Port.

Update: I couldn’t mention this when I originally answered a week ago but MODX version 2.50 adds the DAW Remote function...

REMOTE layer communicates exclusively via USB Port 2 with external DAW software (Cubase, Logic Pro, Pro Tools, Ableton) and can run these DAWs remotely, including reassigning the entire front panel to control DAW plugin synths and effects. A REMOTE layer re-tasks the front panel as a control surface - this is the ideal situation when controlling an external software or software based device.