Function XPLMSetAudioVolume 

pub unsafe extern "C" fn XPLMSetAudioVolume(
    fmod_channel: *mut FMOD_CHANNEL,
    source_volume: f32,
) -> FMOD_RESULT

XPLMSetAudioVolume

Set the current volume of an active FMOD channel. This should be used to handle changes in the audio source volume, not for fading with distance. Values from 0 to 1 are normal, above 1 can be used to artificially amplify a sound.