These two phrases mean basically the same thing. The second phrase is used to strengthen the first. (See: [[rc://en/ta/man/translate/figs-parallelism]])
Here "voice" represents what Isaiah says. AT: "to what I say" (See: [[rc://en/ta/man/translate/figs-metonymy]])
# to my words
"to my message"
# Does a farmer who plows all day to sow, only plow the ground? Does he continually break up and harrow the field?
Isaiah uses rhetorical questions to make the people think deeply. AT: "A farmer does not plow the ground over and over and continually work the soil without ever sowing seed." (See: [[rc://en/ta/man/translate/figs-rquestion]])