Hi,
Yes, double checked, it's all possible right now.
the easiest way is to lock the fsm you want to change the rolloff ( tho why don't you simply change the rolloff during editing?!).
when you have your fsm locked, select the gameObject in question, and drag the audio source component onto the state, and you will be able to choose between "Set Property" and "Get property", choose "set property", and then you can choose the RollOffMode and set it to linear.
But again, why do you need to do that at runtime? when you can simply set it in the inspector?
bye,
Jean