"Pump up" is a recognized phrasal verb. "Pump" has the idea of increase, so you can probably understand why "pump down" is not recognized as a viable expression.

Thank you for your precision Hilary, that help me a lot;)

lower the volume is also used to mean turn down the level of sound reduce the volume would be used if you mean the volume of liquid - reduce the volume of liquid in that recipe for a better result.