Why does it say "la'Rochev bi'Shmei Shmei Kedem"?
Radak: This is like Shmei ha'Shamayim. It is the top sphere that surrounds everything, called Aravos, like I explained (refer to 68:5:2:1). It says Kedem, for Shamayim and its beings stand from the beginning, from the day they are created, unlike earthly beings.
Malbim: The Mizmor concludes like it began "Shiru la'Shem Zameru Lishmo" (verse 5; verse 33 is very similar). Verse 5 says also "la'Rochev ba'Aravos" - He 'rode' in the Midbar to save them via war, i.e. via nature. Now He rides in Shmei Shamayim, a miraculous conduct higher than our Shamayim, which has a natural, ordered conduct. Shmei Shamayim influences on Shamayim; it is a higher level, a miraculous conduct higher than our Shamayim.
Why does it say "Hen Yiten"?
Rashi: It means Hen Nosen (He makes His voice audible).
What does the verse teach about the voice?
Radak: It is on (against) the enemies.
Malbim: It is strong - it is above a natural voice.
