Problem with sounds and rolloff

I have a particle effect prefab which has an audio source set to play on awake. What kind of a number should I use in the rolloff? No matter what I try the sound is always the same volume, more rolloff just makes it quieter at every range. And the sound plays randomly at full volume no matter what the rolloff is.

Is the audio file mono?

The audio files are forced to mono from the audio importer.