This is now august 2014 !!! Almost two years after this problem was reported. I'am trying a CS6 thing and never had this problem until now when I dropped what seems to be a killer issue: 3 different sources of digital files into one sequence. I never used different cameras, but I received some full HD files. One audio only from Zoom recorder (was 1,411 kbps...), one AV Quicktime mov from a Cannon EOS 70 D and one AV MTS AVCHD from a Sony PMW 100.
The report is the same: no problem importing and editing but no way to export audio, neither only in wav or mp3, neither with the video. As people noticed, unmarking "export audio" your video edition was exported as ever.
No hardware issue too, as things evolved form 2011-12 and now I'm using I7 with 8 fast cores, 8 GB DDR3, SSD SATA 3 hard drive only for editing, and a 960 cuda cores Video Card. So, all the previous "weak hardware" and "re install" things were WRONG!
thyl0r's reply is the only correct thing at this issue. I tried others (no re install, of course) and they failed. I made the - simply click "sequence->render audio" - and my despair vanished in less than a minute. This answer has to be incorporated to Premiere's FAQ. Thank you thyl0r whoever you are.