In this passage we have a couple cases of minor variation in the textual history. First of all, in the 1906 LDS edition, the indefinite article a in the phrase “a great tumultuous noise” was accidentally dropped. This edition never served as a copytext; thus the a has remained in the subsequent LDS text. The 1906 loss of the a after “a voice of ” may have been influenced by the lack of a after “a voice of ” both before and after this clause:
Taking the opposite tack, one could argue that the a before “great tumultuous noise” in 𝓟 was an error of perseverance coming from the a in the immediately preceding “a voice of ”. Here the critical text will follow the earliest reading, “neither was it a voice of a great tumultuous noise”.
The other minor textual variant in 𝓟 for this passage occurred when Oliver Cowdery initially wrote small instead of still in “it was a still voice of perfect mildness”. The correction, a supralinear insertion of still, appears to be a later correction since it was written with a slightly sharper quill; perhaps Oliver made this correction when he proofed 𝓟 against 𝓞.
One may wonder here if the original text read “still small voice”. This longer expression that includes both adjectives is found once in the Book of Mormon text:
This longer version of the phrase is also found once in the King James Bible:
The Book of Mormon also has an occurrence of “small voice” (that is, without the still ), so the other shorter expression is also possible:
Thus the Book of Mormon, based on its earliest textual sources, has a single occurrence each of “small voice”, “still voice”, and “still small voice” (but none of “small still voice”).
Summary: Accept in Helaman 5:30 Oliver Cowdery’s correction in 𝓟 of “small voice” to “still voice”; Oliver was very likely correcting to the reading in 𝓞, no longer extant here.