I don't have the greatest hearing after close to 30 years being a mechanic, Some frequencies I can't hear at all
Get the 1-2-3-4/3-5-R drum out of the way by installing the Sun-Gear & Bearing on the Stator, Then slide the 4-5-6 drum/Turbine Shaft into the Stator Tube, May allow you to hear where the leak is.
I've had spotty luck with having the Pump Cover/Stator & Pump Body/Bellhousing machined...So 90% of my 6L80/90's get new OE parts.
What procedure did you use replacing the Stator Support Gasket?