You know that tagline from Alien, don't you? "In space eveyone hears you scream instantly"...oh, wait. No, I guess it's "In space no one can hear you scream".
Yes, sound does not travel through a vacuum (I was being silly of course). Then again, we don't know if space in the Narnia world is a vacuum (heck space in our world, as realized in the Space Trilogy, is hardly a "vacuum", what with all the eldils "filling" space there as Ransom reports:-). But even if there was a "medium" for the sound to travel through in the Narnian world, we already know that even just the fact that it is a flat world would require some kind of magic, so it's already there a priori:-)
--Stanley