they should sound a little like the name they say. (unless they don't say their name)
combee (first guy that I thought of for some reason) can have the cry: COooOooOoOommm
and a staraptor (my favorite pokemon) could have somthing like
StarrrRrrRrrrRrrrRr (rolling on the R's)
on the other hand, a pokemon that only roars like charizard can remain the same as it is now.