Can a person who is not mentally sound vote in these elections?
No, a person who is not mentally sound cannot vote in these elections. According to the given context: "...that is to say, every person who is a citizen of India and who is not less than 2[eighteen years] of age on such date as may be fixed in that behalf by or under any law made by the appropriate Legislature and is not otherwise disqualified under this Constitution or any law made by the appropriate Legislature on the ground of non-residence, unsoundness of mind, crime or corrupt or illegal practice, shall be entitled to be registered as a voter at any such election."