This video, by the way, was made by one of the Duolingo Navajo contributors.
The vowel sounds, broken down:
aa- long /a/ sound, like ah. This is said in the low or neutral tone.
ąą - same sound as aa but nasalized
ą́ą́ - nasalized with high tone. High tone just needs to a recognizably higher pitch than your normal pitch (which is your low tone).