“Listen to the voice.” This is the “pregnant sense” of the word “listen,” meaning to listen to what they say and do it (cp. 1 Sam. 8:7, 22).
“warn, yes, warn.” God repeats the verb warn twice, using the figure of speech polyptoton for emphasis. It means to emphatically warn. [For more on polyptoton and the way it is translated, see commentary on Gen. 2:16].
“the way of the king.” The “way” of the king includes his way of being, the extent of his power and the rights he claims as king, the judgments he makes, etc.