The only two things I can think of is the ‘Play Percentage’ in a Multi-Sound container/event or ‘Probability’ in ‘Trigger Behavior’.
You can add a play percentage by right clicking an audio file in a multi-sound container; a percentage will appear next to all the audio files and you can define how often you want each sound to play. This can’t be controlled with a parameter, however, so the next option might be best.
There’s an arrow/triangle on the left of a multi-sound or event’s properties in the deck window. This is a nob that can have automation attached to it so might suit your needs more. This controls the chance for the whole container but the play percentage on the audio files will still be taken into account. This is good for situations where you want a muti-sound to play only a few times, maybe when the player takes damage, but you have a sound in that container that’s repetitive and don’t want it to play often. The FMOD videos actually cover this scenario a little with their ‘Survival Shooter’.
Anyway, hope this helped!