Cleon
Legend
First off, this should be called out as a sonic effect.
Making it sonic suits me.
Switching the base stat to Constitution would mean it wouldn't need a curiously high Charisma to power the Shriek.
Not that I mind it having a curiously high Cha score to support a special attack. That's what the SRD Krenshar does, after all.
Giving the Leaper a Charisma 10 would suit me as much as switching the Shriek to Con-based.
Let's see what Shade thinks.
Second, I'd suggest probably making this Ex and Con-based. And, finally, that's an odd mechanic. Why not say that a victim cannot be affected by another shriek for a round after a successful save?
Basically, I wanted to ensure that if a victim gets hit with shrieks with different DCs (say, a Water Leaper with Ability Focus (stunning shriek)) they need to save versus the highest-DC shriek.
Admittedly, that's rather an edge case since all the Water Leapers in an encounter are likely to have the same shriek DC.
That said, the "cannot be affected for 1 round after a successful save" approach is an acceptable compromise.
Let's see what Shade thinks. Again.
Oh, and I'm not fond of (Ex) instead of (Su). It is a Magical Beast, after all, so we might as well give it something magical.