e - like e in met and a - like a in father
You do high tones in English all the time. Say these phrases. You’re pregnant. YOU’RE PREGNANT?! The latter is higher pitch. So, same concept for Navajo vowels. It doesn’t matter how high you raise your voice as long as it is distinct from your normal speech.
Double vowels can change the meaning of a word. For example, bito', his/her/its water; bitoo', his/her/its juice. Hope this helps.