are you sure you have the audio codecs for all of these files ?
gspot or avicodec will tell you what format your file uses
most probably AC3 or OggVorbis
gspot:
http://www.headbands.com/gspot/
avicodec:
http://avicodec.duby.info
OggDS:
http://tobias.everwicked.com/oggds.htm
AC3 filter:
http://xvalex.hotbox.ru/programs/ac3...ilter_eng.html
good luck