Hey,
I had this problem too, I've made a few videos with VP already but then one day this happens. Not only did the audio stop, the last clip of my video was missing too even though the clip out point was correct. I tried a few things and I got it to work, I wasn't sure if it was because of what I did but you can try it out.
I saved my work, closed VP, deleted the corrupt file, then uninstalled the LAME.exe
Then I re-installed it, restarted VP and re-exported the file and it worked. If you don't have LAME you can get it here:
http://lame.buanzo.org/#lamewindl
Steph