To R' Nechemia, is it not clearly a, 'mum ovair,' since before time of avodah he will have grown?
H David Levine, Roanoke, VA USA
Shalom R' Levine,
Great to hear from you.
This is a very good point! See the comment of the Mizrachi to Rashi on Shemos 2:6 who tries to deal with your question by saying that Rebbi Nechemiah believes that if Moshe Rabeinu's voice was different than his peers when he was a baby, then we can assume it that when he grows older also his voice will continue to be aberrant, albeit in a new way, which would in fact disqualify him for the Avodas ha'Shir of the Leviim.
Alternatively, see the comment of the Maharsha (Sotah 12b DH Kari Leih) who explains -- unlike Rashi on the Gemara -- that by "Baal Mum" Rebbi Nechemia meant simply irregular and thus deformed, but nothing to do with the Avodah.
I hope this helps to address your pereceptive question.
Warmest regards,
Yishai Rasowsky