Solenoid valve - air popping sound
You are right in that PWM may not work since the solenoid was not specifically designed to be proportional.  Still, it's worth a try to see what you get.  You can also get proportional valves, although those will be more expensive.

One obvious answer is to add a muffler to the output of the valve.  Perhaps even a old car muffler if you have the room, but it shouldn't be hard to make something yourself.