The original text has instances of both which and who as the relative pronoun for humans, although which dominates. Here the 1830 typesetter seems to have accidentally replaced an original who with which; he may have been influenced by the following occurrence of which in “the words which had been spoken by Alma and Amulek”. The 1837 edition restored the original who here in Alma 14:7, although this would have occurred as a natural result of Joseph Smith’s editing for that edition. In 2 Nephi 9:45, this same error (replacing an original who with which) was made by the 1830 typesetter; see the discussion under that passage as well as under which in volume 3.
Summary: Maintain the original who in Alma 14:7: “and also all those who believed in the words which had been spoken by Alma and Amulek”.