I have created a performance where one part does not respond to changes in volume. It appears to be the same if the level is 0 to 127. I am layering various Rhodes sounds and the part that won't respond to changes with the Vol of Parts slider is #46 SuperDynoChorus DL.

Typically, when you see Part not responding to the Fader setting, it has been assigned direct parameter control via an assignment within the Control Matrix. Likely, it's been assigned to the Part's Assignable knob.

Is there a an easy way to make the overall volume of that part lower than the others I am using, just as if I was using the volume slider on that patch as a stand alone sound?

You can either reprogram the behavior of the "Volume" with its assigned controller, or you can delete that control set, in which case, the control over volume reverts to the Part Fader.

Here's where to find that Control Set box...
Press [EDIT]
Press the [PART SELECT x] button where x is the Part in question
Press the lower [COMMON] button
Touch "Mod/Control" > "Control Assign"

You can set the DISPLAY FILTER to "ALL" _ this will show you all 16 Source/Destination Control Sets (four to a Page).
You are looking for the Destination box that says "Volume" - this is where the assignment is taking place. Move the cursor to highlight the DESTINATION box. When you do, the SOURCE box will reveal which Controller is currently responsible for this Part's Volume response. (When direct parameter control is assigned it supercedes cc007, the Fader).

As mentioned you can edit it or touch the Box that is labeled "Delete" (lower right of the screen) to remove this Source/Destination Control Set. The boxes will close rank.

Also, how does one load a wave file into an open space and trigger that file by pressing one key so that it runs beginning to end? It is ~ 25 second recording of a speech that I want to make part of a performance.

See my answer here

Excellent. Will try it and let you know how it goes. Appreciate the help.