[pulseaudio-discuss] The volume control ways in PulseAudio

[Sleep]

I read your post badly and thought you wanted to get the current volume 
for the device, to set the volume I believe you should use pa_cvolume_set().

https://freedesktop.org/software/pulseaudio/doxygen/volume_8h.html#a0777581d85a1d4bf9c831bdacaac51ac

https://stackoverflow.com/questions/23291334/how-to-change-the-volume-in-pulseaudio-libpulse