The air pre-charge affects how quickly the pump comes on when water is drawn. I don't think is the cause of the squealing shower.
I find that adjusting the water flow rate at the shower controls, changing the flow restrictor in the shower head or changing the shower head to a different style will usually stop the squealing.
My theory is that is has to do with harmonic vibrations of the pipes amplifying the sound of the turbulence of the flowing water - but that's just my guess.

What kind of shower valves?

Older style valves can squeal if the valve packing nut is not tight.

I cannot recall a single handle valve squealing, but it is probably possible.

Remove the shower head and then turn on the shower valve. Many times the squealing is caused by a piece of debris that gets lodged in the shower head flow restrictor. If it still squeals, then it is probably in the shower valve itself. If it is a two handle, and it happens when only the hot water or only the cold water is turned on, then it could be a loose bibb washer on one or the other stems. If it is a single handle, you may have to replace the cartridge, depending on the brand valve you may have.

All good answers from above post. Still need to know what kind of valve it is. I had a lavatory faucet squealing. The threads on the brass stemm were worn down so bad, the threads were sharp. Very old stem. I turned down the water pressure to that valve and it went away. Got lucky last week and found an identical valve at a salvage shop. Hopefully it will work.