At the time of writing (September of 2016), the only way to do this in FMOD Studio is to use logic markers on the timeline.
First, lay out all your sound files along the timeline as single sound modules. (To make things a little easier, it’s best to leave small gaps between them. These gaps will never play, but they do make the event editor easier to read.)
Then, place a destination marker (or ‘marker’) at the start of every sound module.
Next, right at the beginning of the timeline, place a destination marker. At the same position, add transition markers to each of your sound effects’ destination markers. Drag these transition markers up and down so that each of them is on its own line. Then, going from top to bottom, give each of these transition markers a different probability. The bottom one must have a probability of 100%, the second-lowest should have 50%, the third-lowest 33%, the fourth-lowest 25%, and so on in that fashion until you’ve given a probability to each of the transition markers at that point.
We call this configuration of markers a ‘logic point.’ Whenever the cursor arrives at this point (as it does at the start of the event) it’ll evaluate the logic markers from top to bottom until one of their probabilities succeeds, at which point it’ll jump to the corresponding sound effect.
Now, at the end of each of your transition sound modules, add a transition marker to the logic point’s destination marker.
With that, you’re halfway done: Whenever the cursor reaches the end of one of your transition sound modules, it’ll instantly jump to the logic point at the start of the event, and then to the start of a randomly-selected sound effect sound modules, with no gap.
All you need to do now is to add a second logic point that’ll handle what happens when the cursor reaches the ends of your sound effect sound modules.
Start by creating a marker at an empty part of the timeline. (This will be the start of your event’s second logic marker.) Then, create a transition marker to this marker at the end of each of your sound effect modules. (Note: Not the transition sound modules. Each sound module should have exactly one transition marker at its end, at this point.)
Now, flesh out your logic marker by giving it transition markers to the destination markers at the start of each of your sound effect sound modules. Assign these markers probabilities the same way you did last time, by starting with 100% at the bottom and working up.
With that, you should have the behaviour you’re looking for.
We are aware that the above solution is somewhat cumbersome. We do plan to implement an equivalent of Designer’s ‘Key Off On Silence’ feature that should greatly simplify setting up this kind of behaviour, but we can’t provide any E.T.A. at this time.