So... Speech talk() was working in ZIM 016. But not in 017 and 018. It turns out that that was when we changed tap() on mobile to default to mousedown rather than pressup. Even just using a mousedown event will not play talk() on Apple devices. This is not a first time interaction issue as we can drag things first and a mousedown will still not activate talk().
So either force tap() on the button playing talk() to be on pressup - set mobileUp:true or use a pressup event.