Try doing a Sequence / Render Entire Work Area (from the menu) and see if that does not fix the sound issue. I have had to do this before for certain files that played OK using Windows player but came into Premiere without any sound until I did a render on it.
Also see this thread for CS6 users having issues with Sony files:
sorry but the icon shown that no sound. the file AVCHD (.m2ts has sound on media player. but on premier it imported without sound.
thank you again
hope to fine solootion soon
Three things to read
Metadata contained in folder http://forums.adobe.com/thread/1015001?tstart=0
Read Bill Hunt on Audio Conforming http://forums.adobe.com/thread/726693
CS6 Bug AVCHD http://forums.adobe.com/thread/1004369?tstart=0
-and LOCK the media http://forums.adobe.com/thread/1077245