XPLMSetAudioFadeDistance

XPLM_API FMOD_RESULT XPLMSetAudioFadeDistance(
                         FMOD_CHANNEL*        fmod_channel,
                         float                min_fade_distance,
                         float                max_fade_distance);

Set the minimum and maximum fade distances for a given sound. This is highly unlikely to be 0 - please see https://documentation.help/FMOD-Studio-API/FMOD_Sound_Set3DMinMaxDistance.html for full details. This will set the sound to 3D if it is not already. You can set a 3D sound back to 2D by passing negative values for both min amd max.