Look at the following functions:

SetListenerGain
0 is not heard
1 is no attenuation
values between 0..1 give attenuation

Also use SetDistanceModel to set the proper distance model.
None
Bypasses distance attenuation for all sound sources.

InverseDistance
Inverse distance attenuation described by the following formula:

Gain = ReferenceDistance / (ReferenceDistance + RolloffFactor * (Distance – ReferenceDistance)

InverseDistanceClamped
Essentially the InverseDistance model extended to guarantee that for distances below the ReferenceDistance, gain is clamped.

But there is more:
SetSoundReferenceDistance
SetSoundRolloffFactor
SetSoundMaxDistance

Do have a read on the manual that comes with the creative sdk. That is better capeable of explaining this.