Here "impure" means the person is unclean according to the Law. God does not allow an unclean person to touch the tithe he is giving to God. The full meaning of this statement can be made explicit. Alternate translation: "when I was unclean according to the Law" or "when the law says I cannot touch it" (See: [[rc://en/ta/man/translate/figs-explicit]])
Here "voice of Yahweh" is a metonym for what Yahweh says. Both statements share similar meanings. They emphasize that the person has obeyed all of God's commands. (See: [[rc://en/ta/man/translate/figs-metonymy]] and [[rc://en/ta/man/translate/figs-parallelism]])