According to ancient Sanskrit, it is pronounced a, but the A sound in ancient Sanskrit is guttural, and it will be pronounced e or o if you are not careful. So some sounds will change. But follow the principle of reading a in ancient Sanskrit.
Amitabha Buddha in Nanwu is an ancient transliteration, and now the transliteration is better.
Namo Armida Paya
or
Namo Amitābha
One of the two is better.