I fear that Portaudio cannot do that.
Also it is highly OS and sound-card dependent.
Anyway, you may take a look at those projects to change the general volume and his parameters of the system-mixer:
For Windows:
https://github.com/fredvs/win_mixerFor Linux:
https://github.com/fredvs/alsa_mixerThe code deals only with the general volume of the sound card but you may get the idea how to do for each elements of the mixer.