No ducking in Resolve 14. That's the way it could work.

Problem: Music should be reduced by x dB in a speaker commentary.
Resolve does not offer a ducking function in version 14.

The following path leads to the goal: (All actions take place in edit mode)
0 Select comment clip
1 Activate music track

Shortcut is missing
Set IN and OUT point at the same time for current (selected) audio clip.
(The existing Shortcut X always selects the video clip in its length).
Please make it possible to set an IN and OUT point on a selected audio clip at the same time.

Shift + O
Cursor jumps to OUT point

CTRL + right arrow
Cursor jumps 1 second to the right

ALT + mouse click
Sets Keyframe At this point, the clip's volume is back to 0.

Shift + O
Cursor jumps back to the OUT point

Shortcut missing Clip Volume can be reduced by e. g. 15 dB *)
Please make it possible to jump directly to the Clip-Volume - Volume field via shortcut and enter a value using the keyboard

Shift + I
Cursor jumps to the IN point

CTRL + left arrow
Cursor jumps 1 second to the left

ALT + mouse click
Set Keyframe
At this point the volume of the clip is still 0

Shift + I
Cursor jumps back to the IN point

Shortcut missing Clip Volume can be reduced by e. g. 15 dB *)
Please make it possible to jump directly to the Clip-Volume - Volume field via shortcut and enter a value using the keyboard

Alt + X
Deletes the set IN and OUT points

With this command sequence, for example, in the Contour Shuttle Pro V2 a lightning-fast macro could lower the music track in a commentary.
In Edius, this works out excellently.

All BlackMagic has to do is create two shortcuts.
The one shortcut sets the input and output points of a selected audio clip at the same time.
The other shortcut allows direct entry into the volume field of the clip volume to enter the desired volume reduction.

*) A possibly better alternative would be an input field for the volume reduction by holding down the shift key (playback head is located in the IN-OUT area), because this could be used to decrease the IN and/or OUT point at the same time.

If GrassValley can do that, Blackmagic should be able to do it.

By the way: The playback head is not positioned directly above the keyframe, but always one frame in front of it. This small bug will surely be fixed in the next release.