I understood a few points.
1) Not all drivers support changing the volume. If the volume can be changed, SetVolume will output: "Get volume: ANY" and output: "Get volume: FULL" when it is impossible.
2) AHI has flexible settings for what the application plays (including the volume), but AHI has no obvious way to change the volume of the entire system. AHI_SetVol() function only works with the fact that I started to play. I may be wrong, but I think I'm right.
3) There is a AHIGlobalPrefs structure which AHI operates. The global volume in it can be addressed through ahigp_ClipMasterVolume. Undocumented.
4) Also need to write a function for converting percentages to decibels. AHI has its own fun understanding of volume referring to low-end layer.
5) I looked at the AHI Prefs source, but until I was able to compile them, this is another obstacle: SetVolume needs some of them.
Thank you for testing, I figured out how to test it myself.