Q: New battery,belt and alt but voltage drops and squeals under load

Hello, It sounds like you may have a problem with the voltage regulator. As you know, when turning on an accessory like the A/C, this puts an additional load on the motor which results in the motor needing to create additional power to maintain the same consistent RPM. This sometimes takes a second or two resulting in a delay in the motor maintaining the proper voltage. The voltage regulator is what maintains this consistent voltage across the RPM range whether the motor is idling at ~800 RPM or at full power at ~6,000 RPM. The voltage regulator is a unit that regulates the charging of the battery by the alternator and maintaining a certain amperage while the engine runs. I would recommend having an expert from YourMechanic come to your location to diagnose and inspect your charging system.