Hugh Nibley explains:
The most common name heard in the Egypt of Lehi’s day was the most common name heard among the Nephites, that of Amon or Ammon (the two spellings are equally common, and Gardiner favors Amun), the god of the empire, who unlike other Egyptian deities never took animal form, was regarded as the universal god, and seems to have been an importation into Egypt from the time of Abraham. His name is very often used in the building of other names, and when so employed it changes its sound according to definite rules. Gardiner in his Egyptian Grammar states:
“A very important class of personal names is that containing names known as theophorous, i.e. compound names in which one element is the name of a deity. Now in Graeco-Roman transcriptions it is the rule that when such a divine name is stated at the beginning of a compound [the italics are Gardiner’s], it is less heavily vocalized than when it stands independently or at the end of a compound.”
The author then goes on to show that in such cases Amon or Amun regularly becomes Amen, while in some cases the vowel may disappear entirely. One need only consider the Book of Mormon Aminidab, Aminadi, Amnihu, Amnor, etc., to see how the rule applies in the West. In the name Helaman, on the other hand, the strong vocalization remains, since the “divine name” is not “stated at the beginning” of the compound. Since the Semitic “l” must always be rendered as “r” in Egyptian (which has no “l”), Helaman would in “un-reformed” Egyptian necessarily appear as the typically Egyptian Heramon.– Hugh W. Nibley, An Approach to the Book of Mormon, 3rd edition, (Vol. 6 of the Collected Works of Hugh Nibley), edited by John W. Welch, (Salt Lake City, Utah : Deseret Book Company ; Provo, Utah : Foundation for Ancient Research and Mormon Studies, 1988), Chapter 6
What are the odds that Joseph would have picked so many names which include “Amon”? Where would he have learnt this would be expected in the Book of Mormon?