I was thinking of some Headcannon VAs for my OC's in "The Reign of King Theta"!  And I came up with these actors (reasons why will be added later):

Alpha-Patti LuPone

Since Alpha's Character and Personality were majorly inspired by Yellow Diamond, I just had to make Patti LuPone, the VA for Yellow Diamond, Alpha's Headcannon VA!  Her voice fits Alpa perfectly: everything from her stern tone to annoyed voice speaks "Alpha" to me!  

Beta-Christine Ebersole

Beta is a LOT Like White Pearl.  Everything From Beta's Calm, Drifty Tone, To His Horrid Short Temper and Quick Disapproval.
Gamma-Roger Craig Smith
Delta-Sunni Westbrook or Sarah Stiles
Theta-Giles Patton

Sigma- A Combination Of Dave Fennoy and Scott Porter.

Phi- Amy Sedaris (Voice Tuned)