Why does the Torah see fit to repeat here "Vayishma ha'Cana'ani ... '?
Rashi: To teach us that what they heard was that Aharon died and that the Ananei Kavod had departed
- leading them to believe that they were now permitted to attack Yisrael.
Seforno: This too, was to their credit,
inasmuch as when faced with an attack, they (faced their attackers), made a Neder and kept it.
As opposed to their fathers, who declared (See Shemos 14:4) that they would appoint a leader and return to Egypt.
Why does the Torah refer to Amalek as "Cana'ani Melech Arad"?
Targum Yonasan: Because he joined forces with the Cana'ani and became king of Arad.