I hear the sounds of the windows openning, and closing, in P3Dv4, so you should too. I can even hear the sound over the sound of the External Power.
But if you want to increase the volume, then you need to edit the sound.ini file, found in the P3Dv4\Captain_Sim\757-III folder.
Edit this section/line:

[Sound.459]
;L13_001
File=window.wav
Id=2213001
Volume=0.10

I assume the number has to be between 1.00 and 0.01, so the current settin is at about 10% of maximum volume.

Note. When I changed Volume=0.10 to Volume=0.90 it sounded a lot louder to me.