I have some problem that some files has no audio days before, so I reinstall a clean Vista 32bit, and test with VLC 0.9.6, 0.9.8a and other players (with codec install if needed).
After lots of trial and error, now I'm sure that version 0.9.8a has a problem dealing with aac encoding file. I tried various a/v files, including avi, mkv, mp4, mp3, m4a, ogg, with combination of divx/xvid/h264+mp3/ogg/aac codecs. All files work perfect on 0.9.6 and WMP.