i had same problem with audio in nxcam,
is your audio set to LPCM?
if yes so that's the problem
with some reason CS4 will play it back distorted
there is 2 solutions: (until it will be sorted)
1- sony vegas
2- set audio to AC3 (dolby digital)
Ive been having the same issue in Prem , took delivery of an nx5 a couple of days ago and its been doing my head in trying to get audio sorted in cs4.
Ive also got a media composer v3 system that just wont recognise the footage at all though not even proxy. so thats another issue to sort. bought the little guy as a tester and back up before spearing in to a 350 as main cam . would like to get the bugs ironed out before buying the 5's big bro.
would have been nice of sony to add this info in the manual pdf that comes with it.