XPLMSetAudioVolume

XPLM_API FMOD_RESULT XPLMSetAudioVolume(
                         FMOD_CHANNEL*        fmod_channel,
                         float                source_volume);

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.