How do I change sound file priority in a unit stack?

[EmperorJello]

I imagine most of you folks already know how to do this, but it isn't in any tutorial I've seen. Basically, let's suppose I have a bunch of units in a fleet stack. Heroes seem to take priority over normal units over SFX used when I, say, move a fleet from one planet to another.

 

So there must be some sort of determinant when two heroes are in the same stack. I have FoC, for example, but the mechanics are probably the same as in EaW. I have a fleet stack with Thrawn on top graphically, but when I move the units, I get Piett's sounds (he's also in the stack).

 

Is there a way to make it so that Thrawn has higher sound priority?