What is the problem?
The audio sampling tracks (green) have no MIDI control capabilities.
In other words, there is no way to use MIDI to control the parameters (filter cutoff/resonance, volume, pitch, etc.) of different audio tracks in real-time.
The only way to tweak/modify a parameter (example: Filter Cutoff) for all notes in an audio track is by selecting the track and then tweaking the knob on the panel. If more audio tracks are selected, all knobs you turn on the panel affect all selected tracks.
There is no way to tweak 2 different parameters on 2 different audio tracks in real-time.
Another limitation is that tweaking any parameter for an audio track can only be achieved while in Audio mode.
If you are in MIDI mode - working with internal synth engines or external gear, the audio tracks are essentially frozen and inaccessible.
For any live performance this is a very heavy limitation.
What should this feature achieve?
I would like to propose the following solution:
- Allow an option to assign a MIDI input channel to an audio track (or just assign channels 1-8 by default)
- Bind the audio track parameters to receive MIDI CC messages on the assigned MIDI channel (or use a predetermined standard CC numbers)
Enhancements to the user experience:
- Ability to simultaneusly control multiple audio tracks - live mix, live modulation
- Ability to do so while in MIDI mode - so no switching between modes all the time
- Ability to integrate other gear to control Play+ (external MIDI controllers, other grooveboxes, DAW) - this will unlock so many new possibilities
- Ability to control your audio mix in real-time without USB audio streaming and a DAW
- Ability to have more people using Play+ simultaneusly - for example: Person #1 can use the Play+ panel in MIDI mode to control the synths, while Person #2 can use an external MIDI controller to control the audio tracks (drums, samples).
For live performances (electronic music) this would allow the Play+ to become “the brain” of the rig, eliminate the need for other grooveboxes/samplers or a DAW, and make the entire setup more compact and travel-friendly.
Make MIDI Great Again! 
Are there any workarounds?
Not really 
There is an option to control an audio track parameter for the selected audio track (while in audio mode) - but this is not really the same thing.
There is an option to use USB audio streaming and then bring the audio channels into a DAW, then apply MIDI mapping there to control everything - but also this require a DAW, and kind-of negates the need for Play+ handling audio.
Any links to related discussions?
The only similar post I could find is this:
Any references to other products?
With the risk of sounding harsh - but any other sampler/groovebox since the 90s.
Modern gear: Elektron Octatrack, any MPC, 1010 Blackbox
Vintage gear: I have had this in my Ensoniq ASR-X-PRO (1998) and my Yamaha RM1X (2000)