The 1907 LDS vest-pocket edition and the 1920 LDS edition (independently, it would appear) replaced the past participle proven with its alternative, proved. In the current text, there are two other cases of past participle proven /proved, one of which has also undergone editing, but in the opposite direction as the change here in Alma 34:6:
The King James Bible has only instances of proved for the past participial form (ten times), as in the following example:
According to Merriam Webster’s Dictionary of English Usage (under proved, proven), both forms are acceptable and about equally frequent in current usage. The critical text will follow the earliest textual sources in each instance. Here in Alma 34:6, proven will be restored while proved will be maintained in 3 Nephi 3:4 and restored in Moroni 8:27. For a list of alternative past participial forms for verbs in the Book of Mormon, see under past participle in volume 3.
Summary: Restore proven in Alma 34:6, the reading of the original manuscript; also restore proved in Moroni 8:27, the reading of the printer’s manuscript (the original manuscript is not extant for the book of Moroni).