I say keep 3/day on the breath weapon. There are a number of precedents (though we might put in a "no more often than every other round" or something). 8d10 sonic damage plus stunning seems a little harsh, so how about 3d6 sonic and stunned for 1d6 rounds if you fail the Ref save?
I don't think we need the "no more than once every two rounds" limiter. The party won't hold back, so why should the monster?
I'll bet this is what you remembered...
Detect Alignment (Su): A grythok automatically detects the alignment of any creature that comes within 60 feet. This ability can be fooled by any spell, spell-like ability, or supernatural ability that masks a creature's alignment (or the Use Magic Device skill, although that requires a successful emulate alignment check prior to entering range of this ability and maintaining concentration the whole time within range).
Yeah, I think that's what I remember. I noticed that some critters in the SRD seem to have all the detect alignment spells as SLAs. We could go that route, too, depending if we want it to be continuous or not. What do you think?
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.