i don't think we ever really learn what they say but i think that all harry says in parseltongue really is "open" to get through the chamber. tom riddle, of course says a lot more words some that we see such as "stop" in half blood prince. i'm sure he also says many things like "attack" to the basilisk in chamber of secrets or "kill" but i don't think we know exactly for sure. i hope i helped : )