1) I wouldn't send another email, simply because using the same method of communication is likely to yield the same result.
2) I would call during a non-Monday, non-Friday weekday (colleges have weird summer schedules) and I would be as nice as possible to whatever secretary is helping you (but don't act like it's out of the ordinary, just appreciative). I would not leave a voicemail.
You're doing the right thing by following up -- it shows professionalism and a dedication to the program. Just remember the summer is weird, and them not getting back to you is not a referendum on your candidacy.
