I’ve assigned a voice to a part as part of a performance, however I’d like to make the voice play with a fixed MIDI velocity instead of the velocity sensitivity the voice was programmed with. I see all kinds of editing options for parts within a performance, but not this. Am I missing it? If so, how would I do it?

Answered in your thread at KC (where you indicated you’re using an S90ES), but repeated here in case you missed it:

Each Part in a Performance can be edited for Velocity Sensitivity. See page 153 of the S90ES Owner’s Manual, Velocity Sensitivity Depth. Velocity Sensitivity Offset, on the same page, interacts with it.

For a Part to play at fixed velocity, set the Depth to 0 (zero) and the Offset to the desired value (127 for maximum).