Current openxcom way of controlling volume is via reserved audio channels.
Out of 8 SDL_Mixer channels, 4 are already reserved (2 for UI, 1 for ambient, 1 for unit responses), which leaves only 4 free channels, which is already too few.
I don't want to reserve a whole channel just for dogfight effects.
If you meant dogfight music, that's even worse, there is only 1 channel for music.
We can of course handle volume also completely differently than today, but that's not something I see as a priority for OXCE.
Your best bet is to decrease the volume of the sound/music files themselves.