In this case, I think the Su ability is a bit more appropriate since they don't seem particularly "magical".
Getting back to the breath weapon, how about this?
Breath weapon (Su): 70-foot cone, once every 1d4 rounds (but no more than three times per day), 4d10 sonic and stunned for 1d6 rounds; Ref save DC X half damage and stunned 1 round. The save DC is Constitution-based.