My phone is either in my inside pocket of my shell jacket or in the shallow top pocket of my backpack, and I can hear the alert without issue. This is because yesterday after hitting the deck pretty hard, I heard it plain as day.
I had it in the backpack because my clothing for the day (vintage ski wear) lack pockets deep enough for the phone. Also, the voice that calls out to you is loud (and bizarrely cartoonish sounding) and was easily heard through the helmet liner/ear muffs.
One other thing, I haven't triggered the alert with any quick stops on the run or at the bottom, I guess I don't ski fast enough lol