Now I know the answer here is dependent on many factors, but can anyone tell me what they have found to be the best (quietest) velocity for a suppressed 9mm? Obviously 147gr moving at 950fps is further below the speed of sound, but I'd like to bump it up to 1050fps if the extra 100fps didn't negatively effect sound signature that much.

Anything that's subsonic. The sonic crack is what makes the difference from quiet to loud. 

I know 1125fps at STP is the magic number, so figure 1100fps to account for variation. However there is talk of sound signature being effected as a bullet becomes transonic flirting with the sound barrier.

I'm wondering if that sound signature is more pleasant at 950fps as opposed to 1050fps with a 9mm projectile, or if it's not discernible to the human ear.

Being close to Mach 1 under actual conditions is not a good place to be.

Shock waves first start to attach to the base of the bullet.

As the velocity increases they move up to the nose and finally away from the nose and are in front of the bullet.

The tiniest shape changes/variations [like knurling] can cause all sorts of problems and degrade accuracy in the transonic range.

The formation and attachment of Mach waves is not consistent around Mach 1.

Mach 0.95 (and sometimes 0.9) are the lower edge of transonic for many shapes.
Blunt bullets likely tend towards the lower while more 'pointed' ones might get away with the higher.

Supersonic flow is very different than subsonic flow.
Actual flow does not occur over the Mach wave.
It is a 'standing' pressure wave that moves with the projectile.

I reload sub sonic 308 for a 20 inch 1-10 Remington 40X.  The 200 grain speer flat base spitzer is very stable and accurate from 900-1050 fps.  I use Trail Boss.  It is noticably more quiet at 950 fps than higher velocity. Remington magnum primers gave better accuracy than most others.  But the 308 still has a fairly high ES compared to a Whisper or BO.