The 1874 RLDS edition accidentally dropped the adverb up here in 2 Nephi 4:35. Its loss seemed obvious since the 1892 RLDS edition put it back, which is unusual since this second RLDS edition almost never intentionally deviates from its copy-text, the first RLDS edition (1874).
The expression “to lift up one’s voice (or voices)” is very common in the King James Bible, occurring 36 times, including 18 occurrences where the following predicate has the verb cry or weep, as in Genesis 39:15: “I lifted up my voice and cried”. (Note the similarity of this biblical expression to the Book of Mormon text here in 2 Nephi 4:35: “I will lift up my voice unto thee / yea I will cry unto thee”.) On the other hand, the King James Bible has no occurrences of the expression “to lift one’s voice”.
The Book of Mormon also favors the use of the up (eight times, of which five are quotes from Isaiah), but it also has two occurrences without the up:
So the lack of the up is possible in the Book of Mormon text. Nonetheless, the earliest textual sources clearly have the up in 2 Nephi 4:35.
Summary: Maintain the use of up in “I will lift up my voice unto thee” in 2 Nephi 4:35.