Let me explain.
In Spanish, articles agree in gender and number with the nouns they modify.
Therefore, we use feminine nouns with feminine articles. We say, for example, “la niña (the girl).”
Some feminine nouns start with the vowel “a” in Spanish. Some examples are agua (water), águila (eagle), aula (classroom), and ave (bird).
The strength of the voice in those words falls in the first “a” of each word.