Used with FSOUND_Reverb_SetMix, this setting allows reverb to attenuate based on distance from the listener.
Instead of hard coding a value with FSOUND_Reverb_SetMix, this value can be used instead, for a more natural
reverb dropoff.
FSOUND_REVERBMIX_USEDISTANCE
-1.0f /* used with FSOUND_Reverb_SetMix to scale reverb by distance */